In a normal light he might scoff and dismiss it only because every time Lucifer's sense of self gets rocked it is not quiet. It rocks through him like a quake and knocks everything in his mind out of place and picking up the bits not shattered and returning them to where they belong is a trial he's not sure is worth it. Mostly, anyway.

But Istredd accompanying the question with kissing does nicely for derailing Lucifer's normal reaction. It's almost like this man knows him, even if this wasn't on purpose. That alone is reason enough for Istredd to be the only one that knows him to this extent, but then that reassurance wasn't really for that. Istredd knows his mind, Istredd knows his true form. While the former began as necessity, the latter is something he'd never share with someone, and he doesn't see an instance where he'd allow another person to witness it.

He offers Istredd some assistance, reaching up to undo the top clasp of his outfit and pull it back.

I wouldn't doubt your skill at being able to achieve that.