The confusion stems from the fact there no APIs in Android that let apps use RCS. Only Google can use it on Android and no other apps can use it. Anyone can make an SMS app. Only Google can make an RCS app.
It is an open standard, meaning you are free to create your own operating system for phones that implements RCS. But Google doesn’t let you use it on Android, so in practice it’s closed.
Plus, Google’s implementation of RCS adds extra features (like encryption) that aren’t part of the standard. So even if you create your own operating system that implements RCS, it will still be incompatible. So that’s another reason it’s not really open.