This is true for some projects, like Chromium and golang, but AFAICT not true for the majority of projects on that page. e.g. the currently most recently updated repo that doesn't have a "mirrored from..." subtitle is [1], and you can go see all the pull request activity on it (though I'm sure there are also lots of code dump repos in that list).

[1] https://github.com/google/trace-viewer

wrong.

https://github.com/GoogleCloudPlatform/oauth2client and https://github.com/GoogleCloudPlatform/kubernetes, just to list two that I deal with.

Googler here.

Depends on the project. github.com/grpc is being developed github first (and accepting contributions), as are several of the projects on github.com/GoogleCloudPlatform.

This is not accurate. From a strict numbers perspective, ~95% are git repos accepting merges and being worked on. ~5% are mirrors.