Looks fairly believable. Most surveys I have seen have tended to put flaccid length average around 3.5"-3.75". In this one the average appears to be less than 3".
I can't see that he offered instructions on how to measure erect length. If he didn't I suspect some guys reported bone-pressed length, others non bone-pressed, some might have measured from the side, etc. Still, an average erect length in the 5.5"-6" range is consistent with the more credible surveys and studies in which results were not self reported.
More evidence that those folks who believe that the average adult erect penis size is 7" or more have an extremely skewed perspective, or suck at estimating length.