I give it a little bit of stock because if someone came on HN and claimed "Larry and Sergey lit a pile of Bibles on fire and professed their love for Satan in front of the company," I'd be comfortable within the confines of my nondisclosure agreement saying that never happened even though TGIF is a confidential meeting. It'd have to be that ridiculous for me, but I'm sure the threshold for others is lower.
If "joebar" here completely made up the question and answer I bet one of the numerous lurking Googlers would have called him on it by now, even though it's a confidential meeting because (a) not everybody cares about nondisclosure to the same extent you and I do and (b) it's not like you're divulging corporate secrets, you're just saying that's a made up story. Pretty simple. "That story isn't true." Done. Legal gray area, but Google is pretty lax on social media until you fuck up.
I should clarify that it's probably a safe bet that the Q&A happened (and, honestly, sounds like Larry) but I disagree with the commenter's analysis.