Being "rooted" can mean different things, so I'm answering in the broadest sense.

You're right of course. You don't need actual superuser access to the live system to make it work (only through recovery). And, as is the case with LineageOS for MicroG, it can already be integrated into the custom OS.

Thing is, when you start doing such things, the next step is often "rooting" the device anyway, so that you can hide what is considered being "rooted" (which can be as little as running any custom OS) from APKs with "root detection" that otherwise refuse to run.