Yep, I agree with you. It should definitely be consistent across the board.

Oh well, maybe I'll get used to the inertia scrolling. I'll give it a shot when they make the system-wide change.

For now though, the different scrolling experience on AMP (and some other pages) is jarring to the point that I don't even bother and I just bounce.

It feels like putting inertia control options in a browser was a big mistake in first place.

There are no such controls. There are just different places where one can scroll. All other browsers (as far as I know, and this includes Safari on desktop) make all of these the same, but mobile Safari chose to make them different.