Rcs isn't a open source but an open standard. Two very different things.
Open standard: anyone can use the standard but could be proprietary/closed source
Open source: anyone can edit/review the code and forket it if they want to.
The issue with RCS currently is that Google won't release the API for it on android and only allows Samsung to use it for their app. Another part is that their encryption is based on signal and released a white paper about it.
Now it's understandable why people would distrust Google. But apple is currently trying to add e2ee to the open standard (google also tried in the past but failed).