That's something Ione is permitted to tell him. She hasn't known the Truth for long; it's not hard to remember how everything looked to Ione-of-two-days ago, to pretend to be that Ione in her own mind, and speak what that Ione would've said. It's not a lie if she tells Keltham that's what she's doing.
So that Ione tells him everything, starting from being a wizard student in Ostenso, taken on very short notice to a seduction mission. Ione has not, in fact, been briefed on everything Keltham's already been told by the Project Lawful, there's a transcript but Ione hasn't seen it and Keltham has held off on reviewing it; neither Ione knows that Keltham has already heard a version of this story from Asmodia. Ione's story is longer, told in private and under less time pressure.
She tells Keltham about how they were forced to sell their souls - probably explicitly in the wake of Ione being oracled, when they realized that might happen to others - that was the first time Carissa failed to sell her soul - yeah, she failed to sell her soul the second time too, though the devil who said it wasn't allowed right then tried to lock in a price of three Wishes for later -