You can't logically disprove, A is necessary by arguing that A is not sufficient.

Of course just dealing with one great mentor won't make you a great programmer. And good mentors know that, and encourage people to get exposure to additional outside experiences.

But that doesn't invalidate what I said. Which is that if you've never had exposure to someone who is good, then you've never had a necessary reality check that you need to really get the learning process going.