With the introduction of LTE, everything from calls to texts have been IP based TCP/UDP/maybe SCTP packets. Does WhatsApp get to file a net neutrality violation because the phone's native SIP client gets priority by the modem/carrier? Does Gmail get to file a claim because SMS messages exchanged through SIP are delivered faster than their push notifications? Does Telegram get to file a claim because you have to pay for a megabyte of roaming costs traveling abroad while you only pay for a single "SMS" despite both being a TCP packet? I don't know. I don't expect those claims to apply.

RCS is the same, in that it's a core carrier feature that communicates between your phone's messaging service and your carrier's infrastructure. RCS' envelope is actually quite similar to MMS' design, except MMS' data transmission still had to be implemented in a circuit-switched way because it came from the 3G era.

Google muddied the water by offering carrier infrastructure (an RCS server) worldwide to any phone that wants to connect to it. It's as if I would host my own SMSC I'd let anyone in the world connect to. It's not the normal use case and as carriers are implementing their own RCS services, I expect this anomaly to slowly disappear over time.

The distinction between third party messengers and SMS/MMS/RCS is a good thing, in my opinion. SMS/MMS/RCS providers need to be able to exchange what is essentially a live feed on a phone number with law enforcement at a moment's notice. Messengers like Signal don't. If third party services would fall under the same category as RCS, it'd stand to reason that the same would also apply in terms of law enforcement orders, and I don't think anyone but the law enforcement agencies would want that.