Humans aren't the only corruptible species. Because he knows exactly where Lucifer's mind would jump to. Other spheres have multiple species too, some just like Abraxas, like vampires and werewolves. There are elves, dwarves, and gnomes in his sphere. It's not so cut and dry that only one type of creature in the multiverse is the problem.

Since Michael seems to think they're all God's worlds anyway, one could argue it would then be all of God's creations are the problem. But Istredd doesn't know that he believes that their God has made everything. No clarified truth. Just a theory like any other.

Remember when I said you're only sometimes annoying? Technically what he said is he likes Lucifer even when he's being a prick, and that was watered down into annoying. Istredd rolls his eyes and just walks away from him because he has to get the book scent. And it probably will look like he's literally following some kind of energy, his sharp eyes looking for something in particular.