How sound is the fatwa that prohibits adding Quranic exegesis to the margins of Mus'hafs?

This statement is incorrect. The proof of its falsehood lies in the practice of the Salaf (pious predecessors) from antiquity until now, who wrote tafsir (exegesis) on the margins of the Mus'haf (copy of the Qur'an). An example of this is what was done in the Madinah Mus'haf, and none of the people of knowledge have ever condemned it.