Edit you have asked this question rather differently in another post.
The workaround for one known issue for a number of devices follows.
The cause is that when you first ask Windows to select a driver, it selects a small one rather than a combination driver which will perform development tasks.
As you work through your instructions have a Windows device manager window open. As you found, as you switch into fastboot mode a 
‘problem device’ may show up.
At that point in time, use Windows update to search for a driver – usually Windows will find it automatically in its search.
In the event you describe, perhaps you could try the Windows update again, at this point where appears.
Once the is resolved you can move on with your install instructions.
It would be interesting if this trick worked, but I am not so hopeful.
Fastbootd seems to be functional in limited circumstances. One known thing is that the sequence of commands must be exactly correct. (For instance a similar thing might happen if the phone accidentally booted into system during the sequence of commands, or one stopped, then went back and retraced one’s steps.)