> NUC is somehow recognized by Windows 11 as being entitled to a copy

There's some form of "BIOS-attached license". Don't really know how it works, but I've seen this for many years. Basically all PCs that have the Windows logo have that, and you can install windows on them as many times as you like, without ever having to enter a license key (I suppose this is limited to the same edition level - I've only ever tried this on "enterprise-level" machines that came with windows pro).

This even works for machines that originally came with windows 8 to install 10, and 10 -> 11. I've never tried "forcing" a win11 install on any machine that came with win8.