They've already established how oblivious Lucifer is. Istredd had to, quite literally, spell it out for him and even then Lucifer didn't believe him for the start.

It's wonders that he's accepted it well enough now.

'Probably' without killing you, he corrects. There's some concern there that he could accidentally do so. Mages are still very human, after all. He'd likely be the first to defend Istredd to anyone else that the man is strong enough, but there's a difference between strength and your body is not stone.

He smirks. He thinks Istredd might regret those last words.