Nobody is arguing that ALL players improve. Pointing out two players who had historically great rookie seasons does not mean that, as a general rule, bigs take longer to develop.
OK, so let's ignore the fact that they were both older and/or more experienced than Ayton. Those are two extreme examples. Some players have outstanding rookie seasons, but most don't, especially when it comes to the bigs.
Shaq was a "one of a kind" player. There has never been someone so big, so strong, yet so quick. He also played THREE years of college ball. Griffin was another freak of nature. He was every bit as athletic (if not more) than a young Stodemire, and I honestly cannot remember another big with that kind of athleticism. I would also argue that both of them DID improve significantly later on.
I think looking at guys like KAT, AD, Drummond, etc is a more reasonable way to look at how young bigs develop. Hell, even if you look at someone like Patrick Ewing back in the day showed major improvements later on. Ewing was 23 when he was a rookie, Robinson was 24.