> A scanner still relies on macOS so, even though it's a virtualised macOS instance running atop Linux (on an Mac Mini), it's still macOS.

Note the parens. It's on Apple-branded hardware, so IANAL, but that's more likely to be kosher.

It does not. As long as you own the physical hardware you are okay to install on virtualized hardware.

The scenario from the post is acceptable only because it is running on a Mac Mini running Linux. If it were running on any non-Apple hardware it would not be permissible -- even if he bought an Mac Mini and never used to attempt to "buy a seat", because the Apple license states:

"The grants set forth in this License do not permit you to, and you agree not to, install, use or run the Apple Software on any non-Apple-branded computer, or to enable others to do so."