I heard they gave free condoms only to the experimental group, and not the control group, so that might be a factor. And I heard that the timeframe started the same for everyone, despite the experimental group healing and not having sex, also putting setting the control group up to fail. It'd be nice to have something to cite though that these flaws in the study are real and confirmed.

As for biological theory of the "how", I'm guessing they would say something like the foreskin trapping more bacteria and viruses, in which case the debate shifts back to consent and cleanliness.