I mean, sure we can put labels wherever we want. I don't really care. MirageOS even has "OS" in its name.
The point is all the other things: total number of lines of code, permissions and security, memory management, drivers, process mgmt, it all changes.
Does it though? And I'd really like to see how you'll revirtualize e.g. an NVIDIA GPU (notoriously context-full piece of hardware), while allowing transparent shared access to it from different applications. Heck, you'll run into trouble with most of PCI-connected devices except for the most primitive once.