I don't think that's implausible, but that misses my main point. If they are officially blaming China, they likely have some solid evidence from inside sources,

I don't think that's implausible, but that misses my main point. If they are officially blaming China, they likely have some solid evidence from inside sources, not just an IP from China. Break-ins happen all the time, everyday, but not everything is attributed to someone because they don't have enough to go on. In a case like this, it could be as simple as a captured email from Chinese intelligence saying "Got those d0x you wanted. lol".