But look, Lucifer is internally lit up with delight about it.

See, his rudeness comes with perks??

He suddenly puts all his force into slamming the nose of the boar into the ground, grinding the tusks deep into earth, and the partially leaping back, partially catching himself with his wings, to land at Istredd's side.

Optimistic of you that she didn't know before. If anyone was going to suss it out, I'd bet on her. You could've just asked offhand with no explanation. But likely even offhand Istredd would've gotten flustered and revealed himself too clearly.