How could a twin aisle aircraft (Boeing NMA) ever have a single aisle (A321) economics?
So it's definitely not ALL marketing spin, but it's also definitely not ALL engineering, either.  

The frontal area is absolutely a major component of the drag on an airplane, but wingtip devices (sharklets, winglets, etc.) can significantly reduce drag, as can better nose shapes, smoother surfaces, and a number of other design considerations.  If you kept all these other things the same, then increasing the frontal area would increase drag and reduce efficiency, but these other things aren't the same in a new airplane.  The MoM proposal would certainly take advantage of all sorts of wing, engine, airframe design improvements that would reduce the drag components, and then you'd gobble most if not all of that up by making the airplane wider.  ;-)

If I'm putting on my "engineer but worked in marketing for a while" hat, I'd say that the challenge put to me is that the marketing people want an airplane that can be operated at what used to be called single-aisle efficiency, but can now be done with widebody seat capacities.  

I'd also contend that of the parameters mentioned: "range, comfort, capacity, and faster turnaround time" the only one of those things that really requires you to think seriously about a twin-aisle airplane is capacity.  I can build a single-aisle aircraft that has every bit as much range, way more comfort, and a much faster turn around time as a twin-aisle aircraft, it just doesn't hold as many people.  ;-)