If the goal is to be fully free from backdoors and development being cutted out at any time, yes.
If Google cuts development of AOSP in favor of some closed-source alternative, the GrapheneOS team could simply continue development of AOSP on their own.
There exists a possible world where a group of underpaid FOSS devs forked Chromium and AOSP and effectively developed it further.
But it is not our world.
> the GrapheneOS team could simply continue development of AOSP on their own.
They won't be able to do so.