Google gets data from the Chrome UX Report (CrUX, everyone can access aggregated data on Big Query), so they'll even get information on performance from pages t

Google gets data from the Chrome UX Report (CrUX, everyone can access aggregated data on Big Query), so they'll even get information on performance from pages they themselves never see, and "the origin" (aka the domain) is the most relevant group for most websites, because they won't have enough traffic on each individual page. If Google only relied on measuring itself, it would just be a cat and mouse game to identify Google's testers and serve them a stripped down version. But they already have the actual real user data.