I wonder how Google would actually implement that, given that "Google Messages", as far as I can tell, isn't really a "platform" (as stated in the linked article) but rather a client for RCS, which needs mobile operator support to work on Android, and to my knowledge does not work at all on iOS.

Link in case anyone else is curious about RCS: https://en.wikipedia.org/wiki/Rich_Communication_Services

Yeah no. Google Messages (almost) always go through Google services, not carriers. They are definitely their own platform.

That's a vast oversimplification and is not really telling the whole story