I think what it's saying is that you use more muscle fibers to move a heavier weight.
Or...maybe that you actually recruit as many muscle fibers around the clavicular head of the pectoralis major when doing less weight on incline compared to doing more weight on decline.
The one sure thing is that they didn't use very experienced test subjects, they didn't use a large number of samples, and there are lots of unaddressed variables. Here is an important one. Among people that have been lifting for a year, how many regularly do incline bench press? Typically, not that many...it wasn't addressed in the study anyway. So what's one of the first things that happens when someone is just learning to lift? They undergo neural adaptations that increase their ability to recruit muscle fibers. Which means if they had more more experience doing flat or decline bench and there WAS a significant difference in recruitment in different areas of the pecs, then this data wouldn't be very significant.
I'm just rambling. I'm a powerlifter and I don't care.