Two, the DMCA seems to not apply here, if this is a Chinese citizen enforcing legal rulings against another Chinese citizen. That is a perfectly valid situation to call the police, in any country. The message getting relayed through a third party (Musescore) doesn't really change it.
Three, this argument is going around in circles, I think a great option would be to contribute to the author's musescore alternative: https://github.com/LibreScore/LibreScore
The DMCA absolutely applies here, since the whole request was for the repository on Github to be removed, and Github is an American website that follows American law and receives DMCA requests.
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.
Also, considering the owner of the repository considers the Chinese state a dictatorship, it's pretty fair to assume "calling the Chinese police on him" doesn't mean anywhere near the same as it means calling the police in Europe or America. This is clearly the worst kind of threat you can make to this developer.
You seem to be giving all the possible benefit of the doubt to the company while giving less than none to the repository owner, even when they provided information about how what they're doing is not breaking the law, and provided a proper way to solve the issue, DMCA. At the same time, the company hasn't provided much.
I don't see why anyone should assume that the company knows anything more than that but is still resorting to threats instead of solving this the easy way.
The accusations don't have a leg to stand on, otherwise a simple DMCA would have solved the issue, period.
EDIT: Also, I don't see how this conversation can continue. If you don't see a problem with calling the CCP police on someone whose only personal information we know is that he's anti-CCP, then I don't think we have enough ethical common ground to even continue this discussion.
However I would still encourage open source authors to be educated on Chinese law and to avoid telling Chinese citizens to do things that could potentially get them arrested in China. If there is a real legal threat there it's irresponsible to ignore it. So in the end it's probably good that it was posted on Github.
Edit to respond to your edit: That's not my view. The issue is that the police have already been called by a different party. (or the equivalent of it, i.e. the company is contractually obligated to contact the government in the event of a violation, under threat of legal action from that government) The email appears to just be informing them of that fact. In my view, that is the only responsible thing to do. The only other option is to just not inform them, and have the police show up anyway.
If this is a real threat then MuseScore really handled it the worst way possible and they should be ashamed for purposefully threatening to endanger a CCP citizen. If it's not, it's just a mafia-style shakedown.
Once again: if you don't see a problem with calling the CCP police on someone whose only personal information we know is that he's anti-CCP, then I don't think we have enough ethical common ground to even continue this discussion.
Edit: I agree with you that if this musescore developer is lying about that, then that would be wrong of them. But I don't think I have enough information to make that judgement. So I think that's where we're disconnected. At least for open source projects, my view is to take any and all potential legal threats seriously until the matter is resolved with the lawyers.
There is also no excuse for sending threats of use of physical force by CCP instead of sending a proper DMCA.
Even if musescore-downloader was doing illegal things (and I would argue it wasn't, since there was no circumvention of anything), the law should be followed by both sides, no excuses. That means sending a DMCA instead of a threat.