How is a conversation in earshot of tens of strangers at a public event a private conversation?

The same way that if you and I were standing in the subway discussing the finer points of <topic potentially offensive to people on the same railcar> or cracking stupid jokes regarding the requirement for some software to have a dongle.

Just because someone in that crowd can be offended by something we say doesn't mean jack shit, unless it is directed directly at them I don't see how it applies to them.

That's entirely a matter of opinion. I think you're wrong. PyCon's code of conduct does too. I think adding a clause about not public shaming is good though. These comments were creating an environment that reasonably made someone uncomfortable. PyCon's updated policy gives people a safe and productive way to deal with it. There's still a risk it'll leak out (as it often does), but it at least lets everyone know what the expectations are.