it is a server cluster - same as
Aegwynn, Bonechewer, Daggerspine, Gurubashi.
All four of those servers have their individual log-in screens same as Barthilas, Frostmourne, etc… but they are all connected.
definition of a server cluster-
A cluster means that characters from any of those realms can interact, be in the same guild, run the same dungeon, raid, share the same auction house, and are effectively the same server, except they don’t have to name-change if there is a naming collision.
About Whitemane Cluster - WoW Classic - World of Warcraft Forums (blizzard.com)