It would probably just use RCS as the backend and have some different functionalities, they could easily just highlight “this person isn’t using Signal so chat features are limited”. Hell, Signal had exactly this when they made the app work as an alternative SMS client. They removed that feature, but it existed previously.
Yeah I really don’t get it. Signal even had something similar. They made it so you could use the app as an SMS client as well. All your contacts would show up and if they didn’t have a Signal account, you could just send them SMS’s. They removed the feature, but they can obviously do it.