I'm just brainstorming here, but ... all groups trained 3 times per week for 6 weeks in the 6-8 rep max range. Did they train to failure? I read/assume that they did. How slow is slow? Anyone look at the full study?
Consider total volume, intensity, TUT, etc.:
The 3 sets slow group likely provided the most stimulus but also likely TOO often. The 3 sets fast group was a distant cousin to CW's method, and the the 1 set slow group was a relative of HIT, but which of these which of these two was more taxing? WHERE is the one set fast group?
"... however, there does not appear to be any additional benefit of training with both three sets and fast contractions."
How did he come to that conclusion? He inferred it, whereas he should have had a 1 set fast group. The study has holes.