That’s because RCS is in a weird position. On a technical, standards-compliant level, RCS is Universal Profile. But Universal Profile was hot garbage and Google Jibe was the only way to make it not utterly suck. But Jibe’s implementation of RCS isn’t exactly Universal Profile RCS, even if it is largely compatible with it. But Google Messenger isn’t even Jibe, it’s Google-specific stuff on top of Jibe, which is Jibe specific stuff on top of Universal Profile. You don’t get the Google Messenger stuff if you just use Jibe as your gateway, a la T-Mobile. The Google Messenger stuff is limited to Google and Samsung, who directly licenses it, and it all goes through Google’s gateway. So, in some sense, it’s still the same mess we had with Universal Profile and its incompatible implementations, just controlled by Google instead of the carriers.
But Google is in large part responsible for the confusion, since they deliberately and maliciously refer to Google Messenger (and Jibe, for that matter) as RCS, as if they’re compliant implementations of the standard, and Apple is the bad guy for not using open standards. But Google Messenger isn’t RCS, it’s RCS + Jibe + proprietary Google features. RCS is Universal Profile and not this other stuff that’s dependent on paying a licensing fee to Google. And Universal Profile is the carriers. So it’s kinda sorta OTT but also kinda sorta not, and a large part of Google’s argument is that it’s not OTT and that it’s the “open” and “standard” way of doing it.