Steam logins are unique. Steam profile names are not unique. Valve further confused things by allowing a custom profile URL - which has to be unique. There’s also a unique SteamID per user, although maybe that’s supposed to be private.
To try and un-fuck their system in the face of Discord competition Valve added friend codes as yet another way to find people. It’s absurd, but marginally better than playing “match the forum avatar to the Steam ID”.