Yes and no, depending on configuration.
As you say, Mill has no privileged instructions, and controls things through address permissions. However, the ISA exposes the full 64-bit virtual address space to the supervisor (OS), so to virtualize the supervisor (and so be able to run two of them) it is necessary to be able to provide distinct address spaces for each guest. As all the space is available, a Mill supporting VMMs must use address space identifiers (ASIDs) internally to distinguish guest address references. This costs hardware, so some Mill configs won’t support virtualization, those for embedded markets for example.
The size of the ASID will limit how many guests can run concurrently.