Didn't they? I mean, I figure everybody read the case studies about page load time and user abandonment. My experience is that those who built a leaner mobile version of their desktop site were also mostly the ones that built an AMP version (WP + "I'll just install a plugin and see what happens" left aside), while those that never cared about mobile users still don't.
Does AMP deliver better rankings? I haven't seen that on sites that I'm affiliated with, but that may be niche specific or the improvement wasn't big enough to go from 4 to 3 etc.
> that never cared about mobile users still don't.
News website still have multiple MB per page and they all got an AMP version which is much more lightweight.
> Does AMP deliver better rankings?
That's a good question. I do know that Google would show news stories with AMP at the top first, but that was part of a different widget. I do remember reading that it helped ranking.