He’s saying the license you paid for allows you to remove Xenforo’s copyright notice, and the software did it because of that.
If you want your own (Copyright myforum 2018 for example) edit the footer to add that. But since you paid for the license that allows removing Xenforo’s name, there’s no reason to put that back. It’s nice to not have it, as spammers search for that to find forums. OTOH Xenforo’s anti spam is the best I’ve ever seen, so that’s not a big concern.