Regarding his definition of ghayb in the introduction, his purpose is to prove that it is permissible to say "so-and-so has knowledge of the ghayb," however the quotes he uses to prove this are decontextualised and may in fact give a very different meaning:
He quotes al-Razi saying: "If it is said: do people know the unseen or not? Our reply is: we previously mentioned that the unseen is divided into that which has evidence to it and that which does not have evidence to it. As for that unseen which has no evidence to it, Allah alone has knowledge of it, and that which has evidence to it, it is not inaccurate to say we know the unseen to which there is evidence."
Al-Razi wrote this in reply to an objection that if in the verse (Qur'an 2:3), "ghayb" is taken to mean the unseen, it would imply its itlaq (i.e. applying it without qualification) to people would be correct but this is in conflict with verse 6:59 amongst others. He replies:
If it is asked: Do you say “a servant knows the unseen” or not? We say: We have explained that the unseen divides into that for which there is proof and that for which there is no proof. As for the one for which there is no proof, then He (Glorified and Exalted is He) knows of it, and none besides Him; and as for the one which there is proof for, it is not wrong for you to say: “We know from the unseen that for which we have a proof.” This sentence is clear in its expression so is not confused [with knowledge of the type of unseen exclusive to Allah]. Based on this reason, the scholars have said: Adducing evidence using the one who was present against the one who was absent is one of the types of evidences.
إن قيل أفتقولون: العبد يعلم الغيب أم لا؟ قلنا قد بينا أن الغيب ينقسم إلى ما عليه دليل وإلى ما لا دليل عليه أما الذي لا دليل عليه فهو سبحانه وتعالى العالم به لا غيره، وأما الذي عليه دليل فلا يمتنع أن تقول: نعلم من الغيب ما لنا عليه دليل، ويفيد الكلام فلا يلتبس، وعلى هذا الوجه قال العلماء: الاستدلال بالشاهد على الغائب أحد أقسام الأدلة
The highlighted part is critical as it shows al-Razi does not allow its itlaq for people, but says it must be qualified so as to avoid confusion with the type of ghayb exclusive to Allah in the manner he showed - by saying "we know from the unseen that for which we have a proof" and not just "we know the unseen." In other words, al-Razi says only when this qualification is made, that we know the ghayb for which there is evidence, are we permitted to say this. Hence, the conclusion drawn from al-Razi's statement in the above article "We are allowed to attribute knowledge of the unseen to humans" is not sound based on what he said.
Mawlana Thanawi in Hifz al-Iman at the start of his answer makes a similar point:
“Ghayb” in absolute terms according to its unqualified usage (itlaq) in the Shari'ah is meant that unseen matter for which no evidence has been erected, nor is there a medium or a means to its perception. For this reason, “None in the Heavens and the earth know the ghayb besides Allah” (27:65), “If I were to know the ghayb…” (7:188) and other such verses were revealed. To call that knowledge acquired through a medium “ghayb” requires a qualification (qarinah) [to explain the intent is that ghayb acquired through a medium], for to use the knowledge of ghayb for creation without qualification may be conceived as shirk [as it may be confused with the type of ghayb that is exclusive to Allah, i.e. that for which there is no evidence] and is thus prohibited and impermissible.
Mutlaq ghayb sey murad itlaqat shar‘iyyah meyh wehi ghayb hey jispur ko’i dalil qa’im neh ho or uskey idrak keliye koi wasitah or sabil neh ho. Isi bana pur “la ya‘lamu man fi al-samawati wa al-ard al-ghayba illa Allah” or “law kuntu a‘lam al-ghayb” wa ghayrah furmaya giya hey. Or jo ‘ilm bi wasitah ho is pur ghayb ka itlaq muhtaj qarinah hey to bila qarinah makhluq pur ‘ilm ghayb ka itlaq muham shirk ho ney kiwajh sey mamnu‘ wan a ja’iz ho ga (Hifz al-Iman p. 14)
'Allamah al-Kawthari, who is mentioned in the above article, also made the same point where he said "knowledge by means of disclosure from Allah is not from the [real definition of] ghayb at all" (العلم بإعلام الله لا يكون من الغيب في شيء), as once there is a means to its knowledge it is technically no longer ghayb.
Al-Zamakhshari also made the same point: "We only know of it [i.e. ghayb] that which we were made to know, or a proof for it was erected for us, and for this [reason] it is not permissible to say without qualification "So-and-so knows the ghayb.""
وإنما نعلم منه نحن ما أعلمناه، أو نصب لنا دليلاً عليه. ولهذا لا يجوز أن يطلق فيقال: فلان يعلم الغيب
The quote that he produces from al-Raghib al-Asfahani was said specifically about the usage of the word "ghayb" in Qur'an 2:3. In this definition, he is referring to the ghayb that is obligatory for everyone to believe according to Qur’an 2:3, not the general usage of the word ghayb. Here is a fuller translation of al-Raghib al-Asfahani's definition of ghayb:
Ghayb is a verbal noun of [ghaba in the sentence] “the sun went out of sight (ghabat)” etc. when it is concealed from the eye. It is said: “Such-and-such was hidden (ghaba) from me.” He Almighty said [quoting Sulayman (peace be upon him)]: “Or is he [i.e. Hudhud] from the absent ones (gha’ibin)?” (27:20).
It [i.e. ghayb] is used for everything hidden from the senses and all that is hidden from the knowledge of people in the sense of [being] absent. He said: “There is nothing of the hidden in the heaven and earth except [it is] in a clear book.” (27:75)
It is said of a thing [that it is] “unseen” (ghayb) and “hidden” (gha’ib) by its consideration with [respect to] people, not with Allah Almighty, for nothing is hidden from Him, as an atom’s weight in the heavens and earth do not escape Him (Qur’an 34:2). His statement “Knower of the unseen and seen” (6:73) i.e. what is hidden from you [not Him] and what you see.
The “unseen” (ghayb) in His statement: “they believe in the unseen” (2:3) [refers to] that which does not fall below the senses (hawass) and the starting point of the intellects (bidayat al-‘uqul) do not necessitate it; and it is only known by the report of the Prophets (upon them peace); and by its rejection the term “apostasy” (ilhad) applies to a person. (Raghib al-Asfahani, al-Mufradat, Maktaba Nazar Mustafa al-Baz, 2:475)
The "definition" that he uses therefore is specifically regarding its use in the abovementioned verse.
Within the second category of ghayb in al-Razi's classification (the one for which there is evidence and can be applied to people), that information which is hidden to some but known to others by sense perception is also included. For example, the Qur'an says of the stories of Maryam, Nuh and Yusuf that they are from the "tales of ghayb" (3:44, 11:49, 12:102), although these stories are only ghayb with respect to the people around the Prophet (sallallahu 'alayhi wasallam), not the previous peoples who witnessed the events described. Hence, Mawlana Thanawi's assessment, that with respect to this kind of ghayb, for which there is evidence and there is a means to its perception, it can be applied to everyone, "because every person has some knowledge which is hidden to another" (kyunkeh her shakhs ko kisi ne kisi eysi bat ka 'ilm hota jo dusre shakhs sey makhfi hey) is sound. Jurjani/Iji make the same argument to disprove the view of the philosophers that knowledge of ghayb is a special quality of prophets.