The Japanese S ranks are getting wiped out not only because of the power difference but because this mysterious ant is being smart and tactical, something they have not seen from the monsters before. They make a point telling you in this chapter that even S ranked monsters are just monsters. They don't approach the battle tactically except in the most rudimentary sense. Even the smarter ones are controlled by their prime directive that tells them to kill human and they take the most straightforward way to achieve that directive. So having never encountered it, hunters aren't prepared to handle it. It's a point that the hired killer hunter from earlier chapters made too, that hunters are too used to fighting monsters that fight more or less mindlessly that they're week against other hunters who have experience fighting humans.
So this isn't that the Japanese S rankers are weak. They're just unprepared against this entirely new threat. And to be fair, most hunters would be unprepared for it too.