Right, and this is not a study, it's public health data. Someone will dig through it to sort out confounders.
Previous infections are one, among both groups. Age - vaccinated people tend to be older and more vulnerable to infections. Other preventive measures - it's possible that vaccinated people are more careful with mask wearing OR that they are more likely to spend time among people and rely on vaccine. The fact that the two groups are likely to socialize with the like-minded group. How likely they are to be tested, especially in view of the testing mandate for the unvaccinated. Etc., etc.
But while the exact number will need to be refined, when seen in context of death and hospitalization numbers, there's really no doubt that vaccines are extremely effective.