Most people don't usually make these kinds of elaborate things up. Prior probability for that hypothesis is low, even if it might be higher for Tuxedage than it would be for an average person. People do actually try the AI box experiment, and we had a big thread about people potentially volunteering to do it a while back, so prior information suggests that LWers do want to participate in these experiments. Since extraordinary claims are extraordinary evidence (within limits), Tuxedage telling this story is good enough evidence that it really happened.
But on a separate note, I'm not sure the prior probability for this being a lie would necessarily be higher just because Tuxedage has some incentive to lie. If it is found out to be a lie, the cause of FAI might be significantly hurt ("they're a bunch of nutters who lie to advance their silly religious cause"). Folks on Rational Wiki watch this site for things like that, so Tuxedage also has some incentive to not lie. Also more than one person has to be involved in this lie, giving a complexity penalty. I suppose the only story detail that needs to be a lie to advance FAI is "I almost won," but then why not choose "I won"?
Most people don't usually make these kinds of elaborate things up. Prior probability for that hypothesis is low, even if it might be higher for Tuxedage than it would be for an average person.
Most people don't report about these kinds of things either. The correct prior is not the frequency of elaborate lies among all statements of an average person, but the frequency of lies among the relevant class of dubious statements. Of course, what constitutes the relevant class may be disputed.
Anyway, I agree with Hanson that it is not low prior probability whi...
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.