It’s daft to put faith in any single study, especially ones that make you and your profession look good. Confirmation bias, or finding the first example that supports your beliefs and running with it, is an easy trap.
Rather than point to this as evidence for why project managers or designers are most important (which is dubious – they’re both important), I do find the question itself interesting. It’s an attempt at a Moneyball, or +/- analysis, for creative teams.
Mr Mollick found that some 30% of differences in revenue between games could be attributed to the producer and the designer alone; and that the lion’s share of this variation was due to the producer. The boring project manager, in other words, meant more to the success or failure of the project than did the flashy designer. Moreover, the effect seemed to persist even as the individuals moved on to other projects, so more than one game could benefit from the same competent producer.
via Business-school research: Game changers (From the Economist).
You can read the actual paper here.