Then how do you explain the "and we don't know why" thing?
Why say that if you don't know that the site has a reputation?
I think
that line can easily have a more innocuous explanation -- "We don't know this site and despite that, we're doing an AMA on it".
What's a lot harder to explain is the infamous Mark taking care of "nasty stuff". Like, if you expect nasty stuff, maybe refrain from doing an AMA? Just maybe?
Though of course it could be that they just expected 4chan-level nasty stuff. Even that should be plainly obvious not to be the case once you visit the site. On the other hand, them expecting to get just another version of 4chan still makes more sense to me, because you can get the "edgy gamer" crowd with that, you can even get some of /pol/, but it's probably even more ignored by the media at large than the current situation. I just don't see the benefit of 8chan vs. 4chan, at all.
Of course, doing an AMA on 4chan is also a tremendously stupid idea and quite nasty. But at least I could imagine the thought process behind that. I really can't for 8chan (and no, not even if the guys behind that were in favour of child porn. But that's a different point altogether).
That still doesn't explain their "joking" behavior while answering disturbing questions on the AMA itself.
Mixing it up with 4chan actually does, I think, because none of the posts in the actual AMA would be out of line there.