I don’t know that Google gets to log your access in that scenario, Plex is just using their login system
Huh? Google would, at a minimum, know what service is requesting authentication, and plex would know which google user account is being used to authenticate. Maybe they hash that information, but why would anyone trust that? Even if you're not breaking any laws with what you're hosting on your plex account, I totally understand why someone might not like the idea of google or plex having data about the identities of users accessing your server and what services are being run from it.