I admit, I haven’t really looked into it. It’s possible Apple implemented new HTML/CSS/JS standards independently, but it’s also possible that Apple continued to backport Google’s changes. Unless they had a business goal of being independent (or NIH syndrome) I would guess that they’d do mostly the latter, but you’d have to go read the code to know for sure.
They are definitely still more related to each other than either is to Gecko (which is to say, not related at all), though.