The supposed shooter took up position on a rooftop only 150 yards away from Trump. I've done a lot of shooting, and 150 yards is nothing. Even with a moderately built 5.56 AR-15, with a moderate scope, it's ridiculously easy, even for a layman, to make that shot. 5.56, in a moderately built AR-15, has 1-2 MOA, which means at 150 yards there's a 1.5 - 3 inch grouping. Furthermore, that round has so little drop at that distance, even in shorter barrel ARs, you can just aim center of the crosshair, without factoring in wind or bullet drop. The Marine Corps (at least it used to, not sure on current training) trains high school kids to make 500 yard shots with iron sites, where the front site post is bigger than the target itself.

There's no way in hell that the Secret Service would allow a rooftop only 150 yards away to be left unguarded or unwatched, unless they're ridiculously stupidly galaxybrain incompetent, or they're in on it.