You're thinking of the way web content blocking works. This is a SMS filter, which is given actual access to the message contents. However, the extension isn't

You're thinking of the way web content blocking works. This is a SMS filter, which is given actual access to the message contents. 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.)