You’re both correct.

RCS is open.

The RCS implementation that Google uses has Google specific extensions that they don’t allow every manufacturer to use (not every Android messaging app or device can have googles RCS) and requires use of Googles own servers.

It’s important to specify which implementation of RCS one is talking about, because it can vary even among carriers. Which is one reason it hasn’t caught on.

It's an open protocol where Google runs the servers for the vast majority. I'm sure Google would accept RCS with the relay operated by Apple.. Not to mention the Google extensions that only provide E2E if using the official Google app. The single reason for Google complaining is that they can't spy on messages between iPhone devices.