"He's just trying to make the immediate area around the main hive-mass a little more toxic so he can get in there and grab it, he's not trying to go toe-to-toe with the mass with a spray can of killer.
Without removing that hive-mass first, fumigating wouldn't get the ones deep inside the nest, and waiting for poison to be carried in - and delivering it in high enough doses - to kill that hive wouldn't be the easiest or most efficient or healthy route.
His goal was the get the hive mass out, get as many of the hornets as possible out of the nest, then mass-killing them would go a lot smoother - they're not gonna scatter, they'll stick to the hive and near the queen(s), and then you can... spray and kill. I'm betting it's not very windy there that day, he's not gonna want what he ends up using blowing around too much."