I can't believe it's come to this...
EDIT: maybe I'm wrong now, as other comment says they changed it a few months back. My last install was somewhere in Dec/2020
Dark patterns abound. It’s horrible, but there are ways around it. For reference, this is in the US (if it matters).
The Mac was connected to the Internet, I had to do a couple of clicks during the setup to create a local account but it was easy enough to do - I had not searched beforehand and I hadn't set up a Windows machine in the last five years or so.
Side note for fellow people willing to run W10 on Mac Pro's, two things: the multi-socket variants won't run on the Home edition, you need the Professional edition, and if you're using Opencore you will need to enable the OpenShell EFI shell as you'll need to run bootmgfw.efi once from the UEFI shell.
AFAIK leaving the ethernet unplugged is the only way to do it now.
This obviously failed as a legitimate Microsoft login, but it did, very handily, offer the option to create a local account instead.