The download page states that because back in October/November BSG still didn't have the Gcam version taken from the 7 series, so technically speaking it wasn't fully compatible. At a later point, when the Gcam got updated to 8.7, it worked out of the box.
Also, that "stable and maintained" is never gonna happen out of the box. In order for most of the stuff to work "properly", it needs a custom configuration. You never use a Gcam without a custom configuration, otherwise, it's pointless.
Those "features" that Pixel 7's stock camera has, might never come to Gcam because certain stuff is encrypted by Google and can't be "added" to the Gcam.