Most IP reassignments I've ever signed have included the clause "any IP developed with the company's resources belongs to the company".
This would include anything created in such a VM, unfortunately. So, if you're not worried about that, it's a great separation of concerns. If you are developing new IP, though, you would want to be very careful about using your work machine.
I'm even nervous about logging into a personal machine via SSH from my corporate laptop, if I'm honest. Might not be enough to legally take ownership, but its probably enough to draw you into a protracted legal battle.
It doesn't sound like he's "creating" anything of value in the VMs. He notes that he mostly uses it for org-mode. It's probably just him needing to access (from his work computer) to notes / agenda info that he would rather keep (e.g.) on his personal Dropbox.
You are right though that anything done using "company resources" may be fair game for a legal battle to determine ownership.
This would include anything created in such a VM, unfortunately. So, if you're not worried about that, it's a great separation of concerns. If you are developing new IP, though, you would want to be very careful about using your work machine.
I'm even nervous about logging into a personal machine via SSH from my corporate laptop, if I'm honest. Might not be enough to legally take ownership, but its probably enough to draw you into a protracted legal battle.