Google doesn’t own the RCS protocol. This is like saying they own the SMTP protocol because they provide Gmail. They are just one company that has implemented the protocol in their default text message app. They built end-to-end encryption into their implementation, which is currently closed source. I’m guessing this is what you’re referring to.
Anyone can implement RCS. It may cost you some money and some time, but it is possible. That’s the difference I was originally trying to highlight.