"It could simply be that the added training volume from the cycling was the determining factor for the different hypertrophy responses."

This is the problem with the OP's study. The volume was low, and they were not trained previously. Also these are male 20-30 year old subjects. Basically any amount of training at less than 12 hours a day with enough nutrition and sleep is going to result in gains. I have never believed that a reasonable amount of cardio does anything to limit strength gains, and it is well known that light cardio can speed recovery. But a real study would need to include a greater age range, trained athletes, and have similar levels of nutrition and volume for all the subjects. For example, would a strength-strength group have the same gains as a cardio-strength group?