No, Apple has a content blocking API. The only thing the app can do is tell iOS something along the lines of “scan incoming SMS for this regex, if it matches, throw it in junk.” The blocker app never actually sees the messages.
Having control of this data is way better than SIM swapping. This seems like a terribly risky thing to install, security-wise.
That doesn’t sound like the same thing as the content blocker api it sounds like it provides plaintext access to sms messages. And it’s enough of a risk that I decided not to install it.
However, the extension isn't allowed to make network requests while it's doing that, and has to pass off a request to contact a server to an iOS service than handles this outside of the extension's control. (I'm not actually sure what restrictions are involved on what Apple will send to your own server, admittedly.)