I don't think von Neumann was saying "some concepts you just need to internalize" at all; his idea was more along the lines of
"The Axiom of Choice is obviously true, the well-ordering principle obviously false, and who can tell about Zorn's lemma?" (Jerry Bona)
In other words, intuitive notions of "understanding" aren't necessarily useful when dealing with "pure" mathematical ideas that lack real-world antecedents. (N.B.: the axiom of choice, the well-ordering principle, and Zorn's lemma are equivalent)
"The Axiom of Choice is obviously true, the well-ordering principle obviously false, and who can tell about Zorn's lemma?" (Jerry Bona)
In other words, intuitive notions of "understanding" aren't necessarily useful when dealing with "pure" mathematical ideas that lack real-world antecedents. (N.B.: the axiom of choice, the well-ordering principle, and Zorn's lemma are equivalent)