I'm not sure what choice they have, it being now, as Lalli said some time ago, too late to leave the Silent World when he thought they should have.
It doesn't seem that the cattank can outrun the horde; and, if they get caught while trying to do so, there'd be no chance to choose ground at that point.
And I don't know that Sigrun's being casual. Some people go dead calm in an emergency -- and in her line of work that would be a useful trait.