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.
Given that their OS requires a pixel phone and google is not releasing
a) updated drivers
b) updated source code for the latest release
their days are indeed numbered.
As for not being a certified android device and being unaffected. That is not true. There will be chilling effects that result in much less FOSS app development for Android, and whether or not an OS is certified is irrelevant in that regard.
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.