The firewall is an inbound HTTP filter, nothing more. It isn't designed to stop outbound traffic.
I know. And it's not even a very good HTTP filter. That's not relevant.
In the context of international relations, how (or even if) it works is irrelevant. In this context, we start from the assumption that it accomplishes the goals that the Chinese government publicly says it does, which is to control the Chinese portion of the Internet for the purposes of computer security, preventing dissemination of "illegal" information, etc.
This makes the significance of the Chinese government blaming attacks on uncontrolled criminal elements operating from the Chinese network space a bit more interesting. Rather than the flat-out denial that one would normally expect from the sort of regime that gets pissy about appearances, it's an official admission that the Great Firewall pretty much can't stop those in China that really want to get around it.