Well, that's partly right.

It was a rocky, wooded island hosting a youth camp for a political party. There were more than 600 people there, and most of them were children or teenagers.

He was disguised as a police officer, and he did gather a small group to him before he started the spree (he killed 2 adults who were suspicious of him before he began), but that's not how he killed most of the 69 individuals who died as a result of the island rampage.

After he started shooting, the people on the island went to seek hiding places or tried to swim the several-hundred meters to another landmass. A few small groups gathered over the course of the event to try to subdue the shooter, but he fought them off, killing a few people in the process.

Ultimately, the people hiding, securing the safety of the younger children, and contacting emergency services proved wise: over 1.5 hours, only 10% of the people on the island were killed.

My point about the crowded square still stands. Denser concentration of people, most of the people are adults, and the shooter is in their midst rather than picking them off one-by-one.