They are still testing linear strength. They are not testing the twisty, non-linear strength imparted by kettlebells.
It's like taking guys who study French and guys who study German, and then testing them at how much better their French got after 8 weeks.
Isn't this problem with the studies obvious, or am I missing something???
Even with the aerobic testing, they are using cycling and running to test this. I'd figure kettlebell aerobic capacity would be different than that for cycling or running as it emphasizes different parts of the physical system.
My kettlebell training makes me strong at moving a weight hanging down from a grip or swinging from a grip.