That's not so remarkable, considering that IBM's VM/370 operating system supported virtualization in the early 1970s (you could run various IBM operating systems in a VM/370 virtual machine, including VM/370 itself):
When you count IBM mainframe Unix vendors like Amdahl UTS, Interactive Systems (ISC) VM/IX, Locus Computing AIX/370, etc, Unix virtualisation (under VM) actually dates back to the 1980s, with the earliest iterations of it reaching back into 1970s (early versions of Amdahl UTS c. 1979 and earlier work done at Princeton to port Unix to VM/370 c. 1975-1976). So Cray doing Unix virtualisation in the 1990s wasn't from that perspective particularly novel. Pretty much continuously from the late 1970s through to today, there has been some variant of Unix running under VM somewhere.
They supported virtualization pretty early in the 90s it looks like, reading the interface for UNICOS on UNICOS