You don't have the data to come to that conclusion. The only way you could draw any conclusion about the safety of the rally in Croatia would be to have details of the Covid case numbers in the people who went to the rally before the event and following the event and to compare them statistically with what was happening in the general population over the same period.
You have no way of knowing whether case numbers are building in the rally spectators after the event but that their increase is being masked by a fall in caes in the general population.