> that the original mainframe hardware has likely been virtualized at this point

The as400 is a mini-computer, the high end of this line overlaps the low end of mainframe.

When I did some consulting work out there many years ago, they had a network of the largest as400's that IBM makes, connected together in one image.

Regarding virtualization: It would have to be on IBM's power processors. IBM does offer cloud services running as400, I have no info on whether Costco is using that or not.