No, it does not. Google’s proprietary implementation implements Google’s proprietary encryption system. RCS does not.

While this is currently only in Google's and Samsung's Messages apps, it's basically just the Signal protocol implemented as an RCS extension. It will most likely end up formalised in the next standard release of RCS.

> It will most likely end up formalised in the next standard release of RCS

Based on what evidence.

If it hasn't been added after all these years it doesn't seem likely it will be added soon.