There's also a difference from the user perspective. Presumably Google will indicate AMP content in the search results page (with an icon, etc), which will let users know the page will open "instantly." If you build a great mobile site yourself, users will never know that until they actually load your page. I can definitely see myself being more likely to open a result that I know will be performant.
Given that recent reporting has shown that ads and tracking are the biggest culprits in page load times, AMP appears to be solving an ad/tracking problem more than a web page problem.
That certainly could be the case, although I'd imagine Google will tread lightly in that regard to avoid anti-trust concerns. They've already taken a policy of using load speed as a quality signal and I wouldn't be surprised if they simply stuck with metrics like that where AMP will do well without showing any sign of favoritism.
