They could've made an open standard that not only websites but other search engines could implement as well, but then again why would they do that? They'd lose a competitive advantage. I think AMP is very deliberately locked in to google.

Any site can implement an amp reader. E.g. Cloudflare has setup an amp cache third parties can use. I believe twitter makes use of this.

This new open standard they are pushing will eliminate the need to even have an amp cache for a site to consume amp, win win.