Attendants are to remain alert at all times even during prayer. This is emphasized more at larger gatherings like memorial and assemblies.
True. However, I haven't seen this during congregation meetings, only during conventions, where the attendants are instructed NOT to close their eyes but to be vigilant & on the lookout for any suspicious activity / threats. Besides, the brother was looking at his tablet half of the time, so it wasn't likely that he was assigned to look out for the rest of the congregation