While I don't disagree that Google are going to be targetting GrapheneOS and other OSes, the decree you're referring to only applies to "certified Android devices" - devices which run a Google-vetted version of Android and that come with Google Play pre-installed. OSes like GrapheneOS are not currently affected by this, as any device running it is not a "certified Android device" by definition.

This is not a reason to sit idly back, of course. GrapheneOS is in danger, as you say - it's just not necessarily from this particular decree.