Not technically illiterate, no. But, he's obviously in an echo chamber, he doesn't read articles, prefers memos no more than one page in length, and less than 12 bullet points (forgot where I read that).
Look, the way someone communicates is indicative of their inner dialogue and resembles their stream of consciousness. Trump's attention span is atrociously short, and he has no patience for nuance or explanation. Combine this with his incredible ability to reshape evidence based on how it effects his self esteem (we all do this to some extent, but Trump raises this to a whole new level) and he may as well be illiterate.