In hindsight, Luffy didn't get "ACoC" because of the coating aspect, which is the easiest/most trivial aspect. All the Scabbards had it. Boa's sisters have it. It's fairly trivial.
So Luffy had "base" CoC control which is 99% of the problem, and was only lacking the other "1%" coating aspect. In hindsight, it's stupid but Oda just hadn't planned it out. But to be fair to Oda, Luffy also learned the internal destruction CoA, which is true "ACoA", not just basic coating.