Edgeworth notices Arthur's expression, the way his amusement fades and his eyes go cold. While he's not quite ready to buy that it's the Atlantis, or at least, the same one he heard about, he knows that it isn't the time to push his skepticism. He can listen for now, and judge what he says later.

"And is that how you discovered your, ah, talents?"

Skepticism aside, it does make sense, within the logic of his story. He can breathe water, Atlantis is, or was, underwater... if nothing else, at least it's thematically consistent.