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.
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.