How can someone be 'wrong' by suggesting a possibility? He never said that all ET civilizations would be hostile. He also never said that any of them definitely would be -- he only proposed that it was a possibility to consider:
"Such advanced aliens would perhaps become nomads, looking to conquer and colonize whatever planets they can reach."
Some of the problems with the arguments against Hawking by those in support of the friendly ETH is that many haven't bothered to read the statements properly (or have gotten their information from misleading sources or incorrect headlines, such as the one this thread references).
He was not speaking in absolutes.