No, this won't cause a fork. To be specific, little forks happen all the time as different clients get information about new blocks at different times. So blockchains are very resilient to that kind of thing. Clients only continue from the longest legit chain they have seen, so whichever side of the fork has more hashpower on it will win in a way that's pretty transparent. So the attacker did cause a fork, so that they could spend coins once on each side, but all clients other than the attackers' would agree on which side was legitimate. So once the attack is over, it settles down immediately.
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!
