The fact is that Facebook is not a media company. In some countries (e.g. most of Europe) where this distinction makes tremendous differences (the media being much more regulated than other for-profit companies), they are pretty adamant about it. Understandably so, business-wise.

Regardless of whether their assumption is true or not, I wouldn't expect the CEO of said company to just come out and say "oh yeah, sure, we're influencers like any other media and we forward fake news too". It's just business sense imho.

As for what Mark truly believes, on a personal level, you'd have to be a close friend, not a journalist recording, to get privy to it.