Yokohama is cut off from the rest of the world, no one is supposed to get in or out unless they have explicit permission and reasoning to do so. However, things in Yokohama and Japan are not unknown to each other. Those in Yokohama know of heroes, and those in Japan know of the organizations and threats in Yokohama. One of those threats being Fyodor Dostoevsky.

The same terrorist who threatened UA, claiming that they have something of his.

'But what could he possible want?' Aizawa thinks as he teaches the class about Fyodor, so that they're prepared, just in case. "And he's the leader of an organization known as the Decay of The Angels, they're terrorists in Yokohama."