It was a fork! That's how the double spends work. The attackers generated a new chain of blocks from just before their transaction, and produced their chain so

It was a fork! That's how the double spends work. The attackers generated a new chain of blocks from just before their transaction, and produced their chain so fast that it became longer than the 'real' chain, causing miners to accept it as the legitimate record of all transactions. As for 'resilient', try telling that to the people who were robbed of their coins because of this attack!