Looks like the ROM update was just to accommodate people putting PC cards in their Mac Pro and needing a boot screen and has nothing to do with secret Nvidia developments.
Except the ROM update... doesn't. It's been examined and it doesn't provide UEFI support for boot screens. As far as we know. It's certainly not doing anything for AMD cards which use the same UEFI boot screen standard as the Nvidia cards.
This would have to be a Mac ROM on the GPU itself. Which would be cool. But would mean Apple wasn't necessarily involved. A future Mac Pro would likely be UEFI GOP compatible, and wouldn't need any special Mac ROMs on the GPU.
Mac editions of cards usually do mean driver support gets added to macOS built in. So if Nvidia is trying to get all the 2000 series blessed as Mac compatible cards, that could explain where the retail drivers have gone to for Mojave.