I tend to think that the increased requirement for intervention has more to do with birthing becoming more prevalent in hospitals than in the home. This being that the stress increase for the woman being in unfamiliar surroundings would be higher in a hospital environment which can stall the progression of labour.

My DP was my hypnobirthing partner so it was his job to keep me focused through everything and without him there I definitely would have lost my focus and possibly panicked.

As he says though, it is only "his observations" so the data is highly subjective and totally based on his own experience. Maybe the combination of him being there with the male partner was the determining factor, not the male partner alone. IMO there are too many variables to make this opinion hold any scientific weight from a research perspective.