They don’t own the standard, but they own the Android implementation of it. Other RCS implantations are hardcoded to not be supported on Android, with the exception of Samsung’s - and they had to enter an agreement with Google (that we don’t know the terms for) to do that, and even then it’s just the Google implementation under the skin. I guess similar to all iOS browsers being Safari underneath.

It’s not open unless you create your own new operating system and implement it that way.

Google’s implementation also adds a bunch of closed-source extensions on top of the standard.