Hythlodaeus mostly just does it for the reaction- even that glance seems to delight him. He knows Emet-Selch can't exactly argue against it.

"Hm. I wonder why you're both forgetting Horos, and not your home worlds? It makes it seem like a dream almost, doesn't it? And yet, I believe that you were there."

His playful demeanour changes to something more thoughtful, as he taps his chin bemusedly. "'Twas mentioned to me that two sides were influencing those who lived in Horos. Perhaps your minds were not exactly clear during your time there. Such interference may have muddled your thoughts."

But that's just a theory based on not a lot of information, so he soon shrugs. "It's my only theory, alas. I cannot say how likely it is for certain."