I don't think that's true though? The OAuth provider knows which third parties you have authorized to have access to your account(s) as well as what information or privileges you've approved for each. And when you land on that screen, they know which third party referred you to them.

In other words, they could definitely highlight or otherwise hint to you which of the Google accounts you've already approved/used via one or more of your authenticated Google accounts.

The provider knows you have given those privileges _at some point_ but it doesn't know if that account still exists or anything about the state of the account.

So, yes, they could do more to highlight _potential_ accounts but it's not the case that they have any visibility into the actual state of accounts.