From what I understand, it has nothing to do with AMP, that is just a coincidence. AMP exposed the inconsistent behaviour in Safari and the Safari developers decided, after revisiting the reason for the different scrolling behaviour, that there was no reason to keep it. Safari just loses its peculiar behaviour in favour of what every other app already does.
Exactly.
Most of the articles written about this aren't properly explaining things - hence the confusion by most people.