+León Castillejos Fernández
Windows NT has been true multi-user for a long time. The consumer editions were not "multi-user" as you say because they used FAT32 as the file system. Windows NT used NTFS, which restricted file-level access and enforced programs to run according to multi-user policy.
XP was capable of using NTFS and properly implemented XP machines took advantage of file-level permissions in this way. Just because people didn't install it properly, does not mean that the OS was incapable of fully supporting it.
That being said, appearance is 9/10ths. If I sit down at a computer, put in my name, and get MY screen, that is multi-user to the eye of the average user. that is all they really want.