Google and Samsung were the only apps to implement RCS (and the infrastructure to support the app) so far.
Nothing Google has done prevents any other organizations that want to invest the time and money from also implementing the standard.
https://www.gsma.com/futurenetworks/rcs/universal-profile/
Apple, on the other hand, doesn't allow iMessage apps or servers that they didn't create themselves. Or even an SMS/RCS app on their devices.