DMCA absolutely does apply here, as Github is an American company and has to follow it to keep its safe harbour status.
There's also no indication that Xmader is a Chinese citizen. The Anti-CPP message on his profile could also mean he's from Hong Kong, Taiwan, or anywhere in the world. The person making the threat seems to be Russian and MuseScore seems to be registered in Belgium.
Even if China could be involved, if MuseScore prefers to solve this using physical force (their words) rather than a simple DMCA, that paints them in an even worse picture than anyone ever in the history of open source.
This was a mafia-like threat, plain and simple. There's absolutely no justification for it, even if it's possible to happen.
