Signal is not an "SMS app". Signal is an end-to-end encrypted messaging app, that just happens to also support SMS. Note that if you use Signal as your SMS app, to send SMS messages, they are not encrypted.
The whole point is that it falls back to SMS if the other party doesn't have Signal, so you don't have to think about it beforehand. It most certainly is an SMS app, and not being able to set it as the default makes it nearly useless.
