If we accept your argument that Google can't measure the speed of a webpage then wouldn't it also follow that Google can't know if the AMP version of a page is actually faster than the non-AMP version?
Google _can_ predict the speed of any webpage to any user. Google Analytics data is enough. Your whole thread is predicated on Google having that one product (crawler). They have everything.
Correction: Google can't predict the speed of a random webpage to a random user around the world.
But you can reasonably predict that pages hosted at widely available CDNs around the world will load faster on average than pages not available.