That is incorrect. It is possible to use RCS on GrapheneOS at least. You need to install Sandboxed Google Play Services, the Carrier Services app, and then go to Settings -> Apps -> Sandboxed Google Play -> Google Settings -> Mobile Data and Messaging -> Phone Number Verification -> Toggle on ‘Automatically Verify Phone Number’.

Also install Google Messages from the Play Store with a dummy account and then log out of that account / delete it.

Here are two screenshots to prove that it is working on GrapheneOS