The article makes the assumption (or at least fails to demonstrate) that the bottleneck is boarding, as opposed to deplaning or cleaning. Other processes which may be done in parallel could just as easily be a bottleneck: food, sanitation, gas, luggage, and such.

And with super-fast turn-around times, airlines might have less time for inspections or maintenance, as we've seen with Southwest's recent problems. (Southwest reportedly has the fastest turn-around time in the industry, in part due to their standardized fleet.)