Pedota:
I think you'll find that in the US there are no immigration checks on the way out of the country, only on the way in (at least that's how it was the last time I visited the place, about 12 months ago). That being the case, there is no way airline staff could have checked to see if the passenger had entered the sterile area as you suggested.
My understanding of this incident is that the Qantas and Cathay flights were both departing from the same gate, with the Qantas flight leaving earlier than Cathay. When the Qantas flight boarded, this passenger simply got up and boarded with the other passengers. Doesn't say much for the vigilance of the staff at the gate or the Qantas cabin staff, nor indeed the intelligence of the passenger concerned!