If you change angles of the Street view, you'll see that there are windows on the backside of that barn.
Nice little cozy spot for a shot.. however, after looking further, I believe that the AGR building would be completely obstructing line-of-sight to Trump's location on stage.. The barn viewpoint is simply not elevated enough to be a viable option, even if someone was positioned on the top of the roof of that barn.
o7 anon