Hi @Bongo welcome to the /e/ forum.
This is a Windows driver issue. Windows has installed a driver for the regular phone and not the “combination driver” which will work in fastboot mode. You could confirm if this is correct by opening a Device manager window and booting the device into bootloader and see if the phone now shows up as a " 
Problem device".
If so Windows update should fix it.
There might be the issue that you need to know how to know you are in Fastboot mode. Did you try fastboot devices (not in instructions) and get “waiting for device” at this point too? [1]
A search for “Fairphone waiting for device” gives 50+ results, you might find more there: Search results for 'Fairphone waiting for device order:latest' - /e/ community
(I moved your thread to a Fairphone section)
Later edit for other users finding this thread.
[1] fastboot devices does nothing to the device but it informs the user if there is full 2 way communication with the device or gives clues if further steps are required, before starting any important step in the reflashing process.