According to comment by head graphene developer on twitter, google devs really like what they are doing and are happy about it.

Apple also recently made changes into their Mac code supposedly to help Asahi Linux team.

Google Pixels generally have the best support for alternate OSes, especially from a security POV. See CalyxOS and GrapheneOS, which primarily/only support Pixels thanks to their ability to re-lock the bootloader.

I doubt third party Android distributions pose any real threat to Google given that 99% of people will stick to the default.

They haven't done that so far, and the predecessors were getting plenty of traction (CyanogenMod, LineageOS). Neither have they controlled the alternative Chromium distributions too much. Nevertheless, the project depends on Google on the long term.