You are right here, the GM could do it, but still the dice mechanics do not suggest doing it...
I don't get it. This is entirely the point of Degrees of Success/Failure. The more DoS you get, the better you do. The more DoF you get, the worse you do. I would argue that it's more straightforward and more flexible than a bunch of symbols.
The phrase you quote has to be read in connection with the rest of my post, so I'll briefly repeat it here. WIth the narrative dice pool you get 3 axis of results: main success-failure axis, secondary advantage-threat axis, you see what gave you the successes/failutes.
When you roll one die (with bonuses, penalties, etc.) you only get one axis, and you lose the information on why you were successful/failed... So, indeed you can rule a lot with just one die roll, but a dice pool of special dice makes it much easier as it gives you a lot of suggestions... I have played with both systems and that's what my experience tells me.
Now, I fully understand why FFG will still use d100 for DH2, and I think it's the correct move for them to make (my 1 cent opinion, of course). They'll have two great dice mechanics and everybody will be happy with the mechanics they choose... It's just that I would pay for a product+dice which ported this setting to the dice mechanics used for WF3, Edge of Empire, or an evolution of them, so just wanted to add my vote to this topic for this reason.