So Google are cooperating with GrapheneOS currently, and sharing the source for the security fixes? Or are they still snubbing it and we have to get them from OEMs? An OEM probably wouldn't be allowed to share the sources to an unauthorized third-party, I think, so straight from Google seems more likely, but with Google not sharing device trees with non-OEMs, it looks out of character for them.
Then again, GrapheneOS shared a lot with upstream over the years as well and have a lot of credibility with Google, whose security team are friendly towards the project. I guess for transparency and clarity it might be nice to specify who is sharing the patches and authorizing an early preview, unless you're literally not allowed to specify.