I don't see any point why they all show up like that. Regardless of whether he knew someone is hiding or not, best keep your exact number hidden bruh. At the very least, his other subordinates standing by don't know exactly where to lock their aim.
It's part of the mindgames.
"I know he brought reinforcements, he's not that dumb. Oh, they showed themselves, but there
must be more nearby. Is that all of them? Does that rooftop hold a sniper? Or are there multiple in those windows over there? We're elevated here, there could be more hiding below." Etc, etc.
Showing a partial hand is an easy way to trick someone who's not cautious or inexperienced into revealing everything they have. This is especially true since One doesn't know all the current Numbers, since he left before some of them got trained.