He doesn't ask why they didn't simply just move away -- by the names of the location, it probably wouldn't have mattered. "Really. Blimey, you managed to kill something that ridiculously immense?" It sounds like a fanciful tall tale honestly, but Jack has seen his fair share of impossible things in the past, so he's willing to give her the benefit of the doubt.

"You mustn't be too overly afraid of the ones flying about here, then, I wager."