A qualified shooter with the right equipment can hit a target more than 2,000 yards (1,829 meters) away. To effectively cover every scenario, you need to turn that distance into a radius, meaning the Secret service needed to cover an area that is effectively more than 4 square miles. Watching every person in that size area would be challenging.
I have no doubt there were drones in the air, but how many may be classified info, as is most likely how much area each drone can cover at a time. It is very possible the shooter could have climbed into position and taken the shot, which I understand was about 300 yards distance, in just a few minutes time between drone passes. Undoubtedly, more info is forthcoming on this event.