You are wrong. On page 4 they say that they want to develop a system to characterize the EFFECT of transmitted road vibration in road bikes and to do that they propose to measure the vibration and rider performance. It is clear they are attempting to measure the EFFECT of the vibration on performance. That thing which brings about an EFFECT is a CAUSE. They obviously are proposing that road vibration may cause changes in rider performance. But they don't keep all the other variables under control by using the same single bicycle and applying different levels of vibration through it. They use three different bicycles and measure the vibration and performance as two dependent variables. They try to keep some other variables fixed, but it isn't possible to know every possible variable. It is a clumsy way to go about the study. First they needed to measure the effect of different levels of vibration through one specific frame (any one) on performance. Then they could have (follow up study) measured the differences in transmitted vibration through different frames once again at a range of transmitted vibration levels. If the performance effect for a given transmitted vibration level was the same on any frame and significant, then it would be possible to claim transmitted vibration as a cause of the performance effect. Alternatively, a statistical design of experiments could have been used to collapse the two experiments into one and to attempt to separate and quantify the many different equipment related causes of rider performance effects. Apparently that would have required an experimental skill level far beyond this team's capability.

Bottom line is that it is easy to design perform an experiment but quite difficult to design and perform a meaningful one.