"Off-the-record" usually means that you and the reporter are the only people in the room. If I give a talk to 10 other people, any of them, journalists or not, can go tell other people what I said. "Off-the-record" has no meaning in that context.
I would say that a public speech is one given in a public forum or at an event open to the general public. In that case, you have no right to keep the press out, and if the event at which you speak is at all important, either locally or nationally, you should probably expect that there will be reporters present.
The Greenhouse issue, though, was not about whether she was on- or off- the record; if there was a group of academics present, she obviously wasn't providing confidential information or anything. I don't know whether the conference was open to anyone or whether it was by invitation only, so I don't know the "public" status of the conference, but I don't think that was the issue, either. It was about whether the discussion was to be televised by C-SPAN, or televised at all.