[thesis]

Just a guess, but this likely has something to do with 10DLC and/or Toll Free Verification and all of the complexities that are being pushed by the carriers for users to register their numbers and even pay to use if you want to use 10DLC.

[Nextgrid]

I believe "SMS support" just means Signal can act as your SMS client using your existing modem & SIM card (something possible on Android), so from the carrier and phone network perspective there is no difference between this and using the stock SMS app.