That does, indeed, explain why he's not surprised. So he's run into someone already that gave him the basics of his previous situation.

"Convenient, that. Saves me the effort to explain a lot of it." Ardbert arches an eyebrow, quizzically. "How much were you told about you and Era?"

Ardbert expects nothing about his potential blossoming relationship with Hades; his mind is violently recoiling from thinking about it, and he can't examine why in the moment, so he'd rather not ask at all.