This article fails to address that most true believers would not see any point to betting on the yes. If Christ returns this year, the world is done and there is no upside to having bet that he would. Temporal things (like prediction markets) will cease to be interesting when Christ returns. Given that, I highly doubt that the people betting yes actually believe that Christ will return this year.
The upside isn't the important part - the downside is. It proves that you really believe what you say.
This happens all the time with countries when they are negotiating. E.g. Russia is pretty unimpressed when countries say they are going to give ukraine weapons unless there is a ceasefire. Its all cheap talk. If one of the countries bought the weapons first (that they otherwise would not of) and then threatens to give them to ukraine, its much more credible since they have sunk money into it.
Or you could look at the animal kingdom with energy expensive mating rituals. The point is to waste resources in order to prove your commitment.
Examples of this sort of thing show up all over the world.