github.community isn't. It sends you to github.com regardless, since their login is there. github.com sees you're already logged in, and redirects back, passing auth.

That suggests it’s bypassing OAuth—with OAuth you’d get a “GitHub Community wants access to X, Y and Z” interstitial.

Most OAuth providers have a flag for first party clients which allows them to bypass the usual authorization dance and just bounce you straight back to the origin.

Unless it's by the same authors as GitHub, and has access to the same OAuth client id/secrets as the master GitHub app.