Maybe the loner hid there much earlier. You are supposed to have people on every “peak”
Advance teams sweep for that and then the obvious security risk locations are secured for the duration of the event.
If that's the case, then the security failure is even worse than it first appeared to be. You're supposed to have total control of the ingress and egress of the area you are in charge of. That a shooter could have already been on that roof prior to the rally starting, and no-one knew it, there is no excuse for that whatsoever. According to the timeline, the shooting occurred within 9 minutes of President Trump taking the stage. That means the shooter was already there well ahead of that time, had his weapon set up and was already taking aim. That means a gigantic security failure, which I firmly believe was deliberate.