This is how the EU works. There are WAY more serious discussions in Brussels behind closed doors[1] than this one, it's not by accident, it's by design.
[1] https://www.yanisvaroufakis.eu/2016/03/30/the-eurogroup-made...
A closed hearing won't be picked apart and (mis)interpreted ever which way, so the speaker can try to get their true point across. The speaker can also acknowledge faults and mention some sensitive issues without making their company's stock ride a rollercoaster.
This is exactly the problem. If he's not able to talk candidly in public, it suggests that Facebook is doing things to which the public would object.
In the same post, this is a very good counter-point, I think. Anything this high-profile will be cherry-picked and de-contextualized a million times. Including in Facebook posts, ironically. Speaking candidly is bad PR for anyone that visible, no matter how virtuous.
To give a specifically British example, the Privy Council holds its meetings behind closed doors, and its (often unelected) members have to take a literal oath of secrecy, yet somehow this aspect of the British constitution does not receive the same criticism as the EU does.