Not every event with two outcomes is 50/50, you know :) And the article does point to both the subject of the bet - unemployment indexes, which are known to all participants and it's kinda the job of the economist to know what they mean - and why Caplan thought he would win. It may be, of course, that he won randomly and his theory is still wrong - but in general people that successfully predict real world outcomes are regarded better in most scientific endeavors than people that predict wrongly.

1) The fact that there are two possible outcomes does not make the odds 50/50.

2) As others have pointed out, he has a history of winning the bets he makes, well above 50/50.

That's more like 1000-1 against.