Imam Kawthari said in his Maqalat:
“Concerning those who criticize Busayri for saying that the Prophet knows the knwoledge of the Tablet and the Pen: neither does all that is hidden, nor does all knowledge reside exclusively in the Tablet. Therefore the denial of the knowledge of the Unseen does not necessitate that of the knowledge of what is in the Preserved Tablet. The denial mentioned in Allahs saying: “He discloses unto none His Secret” (72:26) presupposes exemption of all that is excluded from “His Secret,” signifying the negation of universal disclosure (no one knows all that Allah knows), not the universal application of such negation (no one knows anything that Allah knows). Therefore the meaning is the negation of the knowledge of all the Unseen; not the negation of the knowledge of some of the Unseen. This was demonstrated by Sa`d Al Taftazani in Sharh al maqasid.”
Further, for the Pen, the Prophet said, as related by Bukhari, that during the night of his Ascension he reached a level where he could hear the screeching of the pens writing the Decree, and this stands for his being granted its knowledge. Wallahu A’lam
And, lastly, for the Tablet, then one should know that one of the definitions of the Tablet is the Quran itself as cited in 85:21-22 which Allah has taught the Prophet . Similarly, Allah has granted him knowledge of it as cited in 75:16-19.
Shaykh Gibril Haddad explained this once as such.