I am typing this on a 1987 Apple keyboard on a PC and this numpad functionality works. What I don't know is, is it because of Windows, or because of the ADB to USB adapter?
The numpad keys do have their own scan codes separate from that of the main rows.
Back in 1987, Apple sold a PC compatibility card, and the Extended Keyboard was designed to work with that. So I assume it always sent the correct scan codes.
The numpad keys do have their own scan codes separate from that of the main rows.