SMS (Short Message Service) is an internet protocol for sending (surprise) short messages. It can be used in private communications or public. Any time you find yourself being limited to about 160 characters, the application you are using is likely enough using SMS. The major social media companies use it. As part of their terms of service you have to agree to use it in certain ways. If you don’t, you find yourself looking for another service.
Most social media have some sort of enforcement. For example, you are not supposed to use it in planning or committing a crime. This was a problem for red criminals after the insurrection who had to go for more private and harder to access services, hidden from most users. Still, if part of that service is to send short messages around, they wlll likely use SMS. It is just how you send short messages over the internet. Sure, you can reinvent the wheel if you like, but why bother?
(I guess the answer is that the NSA can recognize the SMS protocol easy enough, and break out the content. The people who are violating the social media terms of service likely want to avoid the common protocols. And yet, your message is likely to pass through multiple machines - Cisco routers, Linux, mainframes, MacOS, Windows, etc... For your message to arrive, there has to be operating system software present to handle the message. If you don't use it, how does the message get there? SMS has to be present in the OS to handle it. There are few alternatives, and all would be designed to be simple and easy to use.)
The problem is that some want to spread false information that kills for political gain. Some people don’t like this. It is against more and more terms of service. I think fact checking public attempts at murder for political gain would be against many terms of service. Fact checking the worst lies would be the least one could do. Spreading the truth would not be blocked by the First Amendment, quite the opposite.
I could see an effort to spread fatal lies over public services resulting in a fact check or worse. Write it in the terms of service. Ban users that persist in it. The problem isn’t with SMS. I would ask if the social media companies should be compelled to fact check, and if the government can force it somehow.
On another topic, Trump has been working towards short term goals lately. Start an insurrection today! Prevent the right to protest immediately! Find the votes right away! Which of Trump's crimes do you see as slow enough that Brandenburg v Ohio would become relevant?