They were in the middle of an auditorium, which is the opposite of private. Regardless of whether it constitutes harassment, the conference has a specific policy against it. Everyone involved said he was wrong and she was right, including they guy, who apologized in person and later in public on HN. She didn't ask for him to be fired.

"They were in the middle of an auditorium, which is the opposite of private."

I do not agree with you at all. If I am at a conference with a friend and we are sitting in an auditorium and he and are I chatting about something between us, regardless of whether you can hear it or not, IS a private conversation. Private here does NOT mean nobody can hear you. It means there are very specific people involved in the conversation.

If the guy, who apologized in person on HN, had said he wasn't wrong and that it was a joke between two friends he would have been lynched even harder than he was now.

I guess it hinges also on the definition of "private".

Is it always the offended party's prerogative to claim offense if, "in passing", they hear something that upsets them but which was (1) not intended for them to hear and (2) not directed at them?

That appears to be the case here - the jokes were not directed at the (now-fired herself) Ms Reynolds.