I did some further reading and the creator of that mechanism offered this explanation at the time:
There are a couple of reasons for the DualDrive having 2 steppers. The first is driving force - pure and simple. We are driving the filament from both sides which means you can have less compression force on the filament. This leads to less deformation of the filament(not getting squished to an oval shape) and allows for a stronger “push” which translates to less jamming. Actually, jamming is almost non-existent. This DualDrive setup is strong enough that it can actually buckle the filament before it skips steps. The second reason was to simplify the parts count as you mentioned. These are small Nema 14 motors so they fit nicely into a fairly small package.
The two motors are adjustable by the fact that there are only 2 mounting screws - one acts as a pivot point, and the other goes through a slot. You simply get one motor lined up, put in some filament and swing your other motor into position against the filament - tight but not too tight. Once it is set you can forget about it as the rollers will now be in a good position to grab the filament next time you load it.
That was an old article so I’m sure someone has done more with that design or perhaps discarded it for something more efficient.