[superasn]

Ok so it won't be perfect but it can be a very good approximate. They can benchmark the site from the user's city and use that value (hey if speedtest.net can have so many servers in every city I'm sure Google can arrange that too).

As far as CDN goes, nobody is stopping you from using it. If you want a site that rival's Google's AMP then you have to use a CDN. The only difference is now it's your choice what you use rather than being dictated to use their non standard tags and technology.