But isn't, because there's a giant icon that clearly Signals it and it pops up a warning the first time you send a message to someone new over SMS, every time. In over 5 years of using Signal, I have never sent an SMS message in the mistaken belief that it was secure, and I typically get hundreds of incoming messages per day and typically send out 100 or so.
'We're taking features away for your security' is a lowkey way of telling users that they're idiots who can't be trusted to operate their own devices. This is really pretty offensive to the people who have been evangelists for Signal for the last 6-7 years.
Did its SMS feature support encryption somehow? Because sounds like a bad idea to include unencrypted messaging in a secure app: it's a giant footgun.