While (I believe) you're right about Apple adding the 3D cube effect for multiple logins, X was explicitly designed to allow multiple concurrent users on the same machine. While many Linux distros did not have that configured by default in the early days, some did and it was certainly doable on the rest.

(That's one of the things that display managers like xdm/gdm/kdm do -- handle starting and stopping of X servers for user logins.)