What I gathered from the video is that he taped the entrances shut at night when most of the bees were inside the hive. Any bees that happened to be outside of the hive when he euthanized it would die eventually from hunger since they didn't have access to a hive anymore.
Bees return to the hive for the night. He sealed the entrance in the late evening (the few bees you see outside afterwards are those who had not made it home at that point).
