I’m a bit confused about the emphasis you put in the quote… GrapheneOS is built on AOSP (the open-source part of Android), it’s definitely not some OS built from ground up (look no further than the various Linux phone projects to see how terrible those are as Android replacements atm).
Technically it isn’t Android, because Google owns the trademark and has some requirements for stuff that wants to call itself Android - it needs to pass a compatibility test and more importantly, include Google Play Services. But it is as much Android as any other custom ROM.