How did they know?

So recently I got banned by the anti-cheat on a server, used old CS:GO alt account with family sharing to avoid ban and continued playing without any hacks at all. Only got to play on the server like 3-40 minutes before the owner came on and banned me. How did he know it was me? Did the server scan my IP adress with the ban list or what? I really don't get it.

EDIT: I made a joke unban appeal, but the owner didn't state how they knew it was me. No screen cap, since I had no cheats. My entire steam profile had nothing to do with my main. Even removed myself as friend before joining. Wot?

ban evasion steps:

1.) remove/backup C:\Steam\steamapps\common\GarrysMod\garrysmod\data and C:\Steam\steamapps\common\GarrysMod\garrysmod\cl.d b
this will remove falco buddies and other stored data btw
2.) change ip and you may want to use a vpn and you live in the middle of nowhere if the owner is geoiping you to be less distinguishable
3.) buy a new acc and never play on it and use it for family sharing or just use the new acc and don't get the benefit of family sharing
4.) join whatever server without cheats
5.) your fine

family sharing can be detected if your banned on the previous account with steam api