PS I studied under one of his protegees. He understood it. Few do. I can do the math, but I don't truly understand it beyond the basics.
What do you mean, “truly understand” ? This is what Feynman was saying was futile. You can understand the rules, and develop an intuition of how they work, but there is no way to delve behind the rules, to see WHY the rules should be as they are. And no Feynman was not “down the rabbit hole”. He was very up front and straight forward in all his expressions, if I may take him at his word. Guys like Wigner and Pauli were indeed “down the rabbit hole”, looking for philosophical or metaphysical implications of QM, which is what Feynman was warning against.