"There are numbers you can't remember if I tell them to you" is not at all the same claim that "there are ideas I can't explain to you."

But they might be related. Perhaps there are interesting and useful concepts that would take, say, 100,000 pages of English text to write down, such that each page cannot be understood without holding most of the rest of the text in working memory, and such that no useful, shorter, higher-level version of the concept exists.

Humans can only think about things that can be taken one small piece at a time, because our working memories are pretty small. It's plausible to me that there are atomic ideas that are simply too big to fit in a human's working memory, and which do need to be held in your head at one time in order to be understood.