I think you can't explain something in simple terms if you don't understand it, is true, but the other way around isn't entirely true, or at least it's a bit more fuzzy.
Often, it takes a lot of awareness of what are the common mental models / mental blocks other people have when learning the concept you are trying to communicate. You have to structure things as a series of strategic progressions before tackling the most complicated form of something, all of that is more the art of teaching ( which of course requires good understanding )
Of course, if someone can do that, it's a brilliant proof they do understand something.
If they can't do it, then it can leave you with doubt what someone else understands. Which in Apples case may be considered entirely unacceptable.
Often, it takes a lot of awareness of what are the common mental models / mental blocks other people have when learning the concept you are trying to communicate. You have to structure things as a series of strategic progressions before tackling the most complicated form of something, all of that is more the art of teaching ( which of course requires good understanding )
Of course, if someone can do that, it's a brilliant proof they do understand something.
If they can't do it, then it can leave you with doubt what someone else understands. Which in Apples case may be considered entirely unacceptable.