I'm not super familiar with the concept (and I'm too lazy to look into it TBH), but I think the would-be winner posted the private key before enough (any?) blocks were mined, and the thief posted a transaction with a bigger fee, and the thief's transaction was in the block that got mined.
> https://bitcointalk.org/index.php?topic=1306983.msg64535839#...
I'm not super familiar with the concept (and I'm too lazy to look into it TBH), but I think the would-be winner posted the private key before enough (any?) blocks were mined, and the thief posted a transaction with a bigger fee, and the thief's transaction was in the block that got mined.