Sure, but the point is that with futarchy as proposed you don't actually have a prediction market on whether the policy is good, you have the government looking at the delta between markets for and against a policy, and the one for the policy which isn't enacted gets voided so anyone manipulating its price loses nothing.
Big Markdown shorts the "No Markdown" policy, it doesn't get implemented and they get their short positions back again.
There's really no prediction market incentive to bet against them: so long as they keep pumping money in to move the rate, none of your bets on the correct rate of GDP without Markdown will ever get paid out on. They only need to bid it one basis point below the price of the With Markdown GDP futures contract to get their policy, so even if you have deep enough pockets to outbid them and good reason to believe Markdown has no effect on GDP, you'd be risking a lot (GDP is pretty volatile and expert forecasts are regularly more than a basis point out in either direction) to win very little if they didn't get their way.
Big Markdown shorts the "No Markdown" policy, it doesn't get implemented and they get their short positions back again.
There's really no prediction market incentive to bet against them: so long as they keep pumping money in to move the rate, none of your bets on the correct rate of GDP without Markdown will ever get paid out on. They only need to bid it one basis point below the price of the With Markdown GDP futures contract to get their policy, so even if you have deep enough pockets to outbid them and good reason to believe Markdown has no effect on GDP, you'd be risking a lot (GDP is pretty volatile and expert forecasts are regularly more than a basis point out in either direction) to win very little if they didn't get their way.