This is most likely misconfigured rev dns.

Somebody used to host ann FTP for them under that IP address, then that IP got reassigned, but the rev DNS records don't reflect that.

We had the same problem. As far as I know, the story there is that when the building I live in first got internet access, they allowed residents to pick cutesy nicknames for themselves, and they'd get an actual domain pointing to their IP, rev DNS, DHCP hostname, appropriately configured search domain and all that JAZZ, because local ISPs were fun like that.

The residents have changed, the IP addresses became (somewhat) dynamically allocated, but the rest of it kept working until the switch from 100mb/s to 1gb/s in late 2023, when it all broke.

Until then, you could get a strange hostname after the at sign in your Mac OS / Unix terminal if you plugged your computer directly into the ethernet socket you had on the wall.

As an aside, until that moment in late 2023, you could track me down to a pair of floors just by looking at what my rev dns said about me. Happier times.