He says that his work is virtualization systems, so the install of (e.g.) VirtualBox itself is not "personal software." It really comes down to whether or not you consider the guest OS image running in the virtualization system to be "installing personal software."
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.
