This is just not true for a multitude of reasons.

The obvious being that most other custom ROMs are not actually degoogled (ie proprietary blobs, still communicating with google) and/or they try to do it in ways that provide no privacy benefit such as replacing google with MicroG which has the exact same issues (ie privelged access to your phone).

The majority of these custom ROMs break any semblence of their security model (rooting, not locking the bootloader etc) to bring these privacy measures, which makes them almost moot in and of itself.

Also purely focusing on removing yourself from Google (especially while ignoring all of AOSP, and to an extent Android in general, is Google centric) when there are tons of other large data gatherers (many of them with far less recognizable brands) that your probably ignoring is useless.