That's sort of where I'm at. I think the kid was so simple, so basic that he basically hid in plain site.
There was no online activity warnings for feds to pick up on. He used his dad's rifle, he didn't talk to anyone and say anything.
When he scoped it out before the event - yeah, people saw him but just assumed he was a dumb kid trying to figure out where he could see the event from without going in. Plenty of other people did the same, so a nerdy looking kid looking around didn't draw much suspicion.
He walked up, literally, to a building full of cops (probably unknowingly) and saw he could climb to the roof via an AC unit or something, so he didn't use the ladder he brought - and he saw that the roof was completely unattended.
It was such an easy shot, he was mm from making it without even using a scope. I believe the snipers seeing him didn't take the shot because it was a building full of cops and they weren't sure yet that he wasn't one of them until he opened fire.
The entire thing looks like incompetence and complacency and poor communication between branches of LEO, all exposed by a simpleton who just didn't give away the signals that normally feds catch before the plot.