Obviously 8.1 in that case was referring to Windows RT 8.1. And the driver is digitally signed by Microsoft, so not sure why we need your driver spiel.
The drivers WTWF is complaining about don't work on WindowsRT, 8.1 or not. The whole "Windows 8.1" thing is a red herring. The reason the drivers don't work on WindowsRT is because only Microsoft signed drivers will load on WindowsRT (this is a policy that can't be turned off, and affects only the tablets, not the PC editions of Windows). They didn't load on WindowsRT with 8 and they won't load on WindowsRT with 8.1.
And no, the drivers WTWF is talking about aren't Microsoft signed drivers because Microsoft doesn't make NICs. They are probsbly Broadcom/Verisign signed, not Microsoft/Microsoft or Broadcom/Microsoft signed, hence why they don't load on the MicrosoftPad.