I think there is a fundamental misunderstanding here.

A multiuser system is a system where multiple users are logged in at the same time and ussing the computer.

So a multi-user desktop Linux would be a computer where multiple people are logged in each with their own desktop session on the same machine.

That was the way unix was first used, a big computer somewhere with multiple client terminals connected to it all doing their own thing. This is the environment x11 came about as well.

Nowadays even if the computer is shared by multiple people each with their own account only one of them is using it at a time.

No. gyulai complained of graphical login managers and advised to set up automatic login. Multiple users sharing a computer with their own accounts would use the login manager for account selection.