It's not exactly a catchall; it's per user, and you can also set up rules to route to different folders based on receiving address. If there are 2 users 'joe' and 'bob' on the domain 'foo.com', joe can use any '* @joe.foo.com' address and bob can use any '* @bob.foo.com' address. Fastmail pricing is per user.
Individually, it doesn't seem convincing, but if you find multiple instances (individuals), in their own different styles, and all the addresses are in the same mailing list... I think it can have significant weight. Alternative would be what? Someone framing the service provider? Or joe and bob and maybe others staging it? Possible, but frankly, not very likely. I think the service provider would have at least have some explaining to do.