No private key was posted too early. What happened is the person who spent all the computing power to brute force the 66 bits broadcasted, naively, a transaction to send the 6.6 BTC reward to his wallet. However, when doing so, the public key is by design revealed on the blockchain. Someone's bot whose sole purpose is to steal this puzzles rewards was monitoring the blockchain and spotted the transaction before it got confirmed (on average confirmations occur every 10 minutes), then it processed the now known public key from which the private key can be recovered in 2^33 operations (2^(n/2)), then crafted another transaction to send the reward to his wallet, with a higher fee, so his transaction got confirmed, instead of the discoverer's lower-fee transaction.
This is a well-known attack. The discoverer was sophisticated enough to brute force, but not enough to know about this risk :)
This is a well-known attack. The discoverer was sophisticated enough to brute force, but not enough to know about this risk :)