Edit: Or at least your opinion on why they would do that.
I have no opinion about why content was hidden. At nearly any other company the same content would have always been secret.
If his cited sources (which presumptively were somewhat important to the point he was making) should (or would have, which may be subjective. I don't know because I'm not a Facebook employee and have no direct knowledge of the now-secret information) have been secret, then it stands to reason that it would have been more difficult for him to make his point.
If that's the case, it sounds like they've changed an entire policy organization-wide to stop more posts like this, or they've made no global policy changes and selectively hidden information that would support his point. Either way I'm curious about this culture of openness and what defines its limits.