[TL;DR keywords in bold]
I find your hypothesis implausible: The game was not about the ten dollars, it was about a question that was highly important to AGI research, including the Gatekeeper players. If that was not enough reason for them to sit through 2 hours of playing, they would probably have anticipated that and not played, instead of publicly boasting that there's no way they would be convinced.
Maybe they changed their mind about that halfway through (and they were particularly resistant to the sunk cost effect). I agree that's not very likely, though (probability < 10%).
(BTW, the emphasis looks random to me. I'm not a native speaker, but if I was saying that sentence aloud in that context, the words I'd stress definitely mostly wouldn't be those ones.)
Update 2013-09-05.
I have since played two more AI box experiments after this one, winning both.
Update 2013-12-30:
I have lost two more AI box experiments, and won two more. Current Record is 3 Wins, 3 Losses.