For people who were used to Ventrilo and mumble and teamspeak, the concept of "server" is well understood and Discord is essentially a voice/chat server as a service with unified identity.
In discord it's more like a slack workspace, right?
The people "hosting" a discord "server" are not actually running hardware anywhere that runs the server?
But yes, a Discord server is equivalent to a Slack workspace. It's just inherited terminology.