I did have the Ethernet cable unplugged to prevent Windows 10 from detecting no drivers installed which it will download/install. I started to try again with the Ethernet cable plugged in but Windows started downloading an Nvidia driver so I aborted by unplugging the Ethernet cable.
That's my workflow too
Will add some kind of "has internet" check for the signing operation.
Edit: Just tested, no timestamp is needed for the driver installation to succeed, so next version will simply remove the timestamping and will work offline
Wish I could figure out what the NVIDIA driver installer does when running, to tell Windows "Hey driver install in progress, don't download"
Edit: apparently it has no such feature. At least not when sitting at the start of the install "select if you want GFE or normal", as soon as I connected the LAN cable on a fresh install, Windows Update would download and install the NVIDIA driver