Makes sense, but it's doubtful that unvaccinated are any more careful than vaxxed.
Actually, this is a known phenomenon, that sometimes when measures are taken to
decrease accidents, the result is an
increase in accidents because when people feel they have extra protection, they may respond by taking extra risks. For example, after requiring kids to wear bicycle helmets, or knee pads on skateboards, they may behave less carefully. So similarly, those who've been vaxed may be more likely to attend large social events.
It's a real shame that they don't take into account whether the unvaccinated we're previously infected, as that could explain why more vaxxed are getting infected than unvaccinated.
They did take this into account, by not including those people in their study. "Previously SARS-CoV-2 PCR-positive individuals were excluded." This allowed them to more clearly focus on the effect of the vaccine, rather than previous infection.