He mentioned that, actually. He said they talked about Sally-Anne Perks and Hydra told him she'd entrusted Sally-Anne not just once but twice with information that could have been very damaging. Once was telling our Dora about Bella's visit not too long ago. The other time was warning Luna Lovegood not to state too openly that she thought I was alive, exactly because Bella was denying it left and right.

And he'd hardly include that in the letter if he were writing under duress. There'd be no need to implicate Luna in anything.