Yes, Google can direct its own users to the cache, and others can also link to the cache. But if you or someone else link directly to the page, it is not served

Yes, Google can direct its own users to the cache, and others can also link to the cache. But if you or someone else link directly to the page, it is not served from Google's cache. Google would not be able to track those visits except for the fact that the page also loads content (at the very least, the AMP JavaScript) from Google's servers. AFAIK, it is not considered "valid" if you serve the JavaScript yourself.