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.

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.