The problem is that the phrase used in Popek's paper is "Conventional OS". This was the 1970s. What's now considered a "Conventional OS" was not conventional back then.
For instance, everyone would agree Linux is a conventional OS but Linux provides a kernel module (KVM) that allows for virtualization driven by userspace.
It refers to a relatively obscure set of formal proofs and doesn't have anything to do with what most people explain it as.