
No, I'm pretty sure MOST of them are there. At least most heroes from that region. They figured out which hero was with the villains because he was the only one left behind in a certain city, for example. It was also implied a few times that civilians were confused that the cities were mostly empty (in terms of heroes) and I'm pretty sure Gigantomachia wouldn't have attacked, like, over ten cities without a single hero doing anything if hundreds or thousands of them were still in there. They even called heroes in training, not even experienced ones, just straight up first year students with newly acquired licenses, to fight against the League of Villains. They're in a pinch.

You can read Uraraka saying "Weren't nearly all the heroes assigned to this mission?" in one of the last panels of chapter 282. If you don't want to believe that most heroes are there, it's on you. If you think it doesn't make any sense, it just proves my point of it being badly planned. Anyway, have a good day, I won't reply anymore.
Ok but how didn't ANYONE think it'd be a bad idea to send most heroes to one place and leave the majority of the citizens unprotected? I do understand the importance of this mission but they should've planned it better.