Practitioners have a problem recruiting subjects. There is often a tradeoff between applying more rigorous experiment design and using convenience sampling (students) versus sacrificing controlled environments (so that professionals would actually join the study).
It's easy to condemn work like this but there's no other option. In this case the researchers chose to replicate a study (which often risks similar ire for telling us nothing new) with a commendable level of rigour and have provided more evidence that, for the scope of experiments we can construct that TDD is probably no different to TLD when using a population of relatively unqualified developers (students).
As to the problem being trivial, what else can be done? There's a finite time you can ethically expect participants to give to you, even if you pay them. If anything the criticism of this work is better directed at the limitations academics are forced to bear.