Going by the rules in the book a normal move doesn’t require any check. (Subtle and Bold moves do but that is all specified).
From the example and Nathans comment I would say that he intends it would be a difficulty 1 test to move into a contested zone.
Difficulty 0 as a default, then modified by 1, either from the PC, their asset or a scene trait (such as Dark in the example on pgs 174 & 175). Otherwise the only way to fail is to roll a complication and why bother?
I can see the benefit if it is thematic for a scene to restrict movement. The example specifies it is a back alley, so Kara holding the thugs off at the entrance makes sense to me. If it is just a contested zone I probably wouldn’t bother.
If you wanted to apply this to the duelling example I would say it would also be Difficulty 1, using the knife as an asset to increase the difficulty.
Given this would apply both ways in the duel and it could be argued that subtle & bold movements should also suffer the same increase as a GM I would personally just drop it so as not to artificially increase the overall difficulty everyone faces.
But then I never got on well with duelling combats, so mostly ran skirmish or contested rolls when I did them.