It says they use site speed in ranking, and, well, I'm sure it would come as no surprise if Google's (largely) having served a page/site increases the speed just enough.

to put it as plainly as possible, it's absolutely hilarious that tech people buy into these insane myths about AMP, there's a reason why no serious antitrust person brings it up, it's fighting on Google's territory - it's a wide open standard, used throughout the industry, formed in response to proprietary solutions designed to tax suppliers by Facebook and Apple, immediately and fully shared with competitors.