I can't speak to beef, but there is more to Android security than what Google provides. That's what Graphene is for, to make Android even more secure through hardening various attack surfaces and introducing other completely new security features. If that weren't the case, Graphene wouldn't be necessary on Pixels because Google does monthly security patching for them.

It's also at the firmware level, which Google does not provide except for on their own hardware, and on top of that Google phones are some of the only ones capable of providing some security feature at the hardware level. This seems to be the main thing the Graphene team is trying to point out.