Looking at the GE image again, there appears to be a low-roofed connection between the buildings - maybe 8 feet tall judging from the doorways.
Lots of wood pallets on the loading dock. He could have dragged one of those over to the 8-foot tall flat roof and used it as a ladder and then pulled himself up. And then go up another 3 feet to the gently sloping roof. Two trees are also between the sidewalk and the flat roof section so he could be mostly hidden as he climbed up.
But it sure seems he must have planned it out to some degree, and knew (or lucked out) how the stage would be set up.
This whole thing would be understandable if it was a highly elaborate plan. This was about as simple as it gets.
On the other side, the active President has significantly better security than a former President/candidate. Some of that is a matter of resources.
I don’t blame the local police for not acting quickly when told there was a man on the roof with a rifle. They may have assumed that every roof had a man with a rifle, because many of them did.
SS officers on the stage did a great job. They were covering him within 1-2 seconds. But it did seem that they dithered in some confusion once he was in the truck about what to do and where to go. But, he was in level 4 armor.