They're not, but they can be ministry of their terms and conditions, which don't allow blatant propaganda posts (regardless of whether it's from a citizen or from a government representative) that claim victims of forced sterilisation are simply emancipating and choosing to have fewer children as an explanation for dropping birth rates.

China is free to publish its nonsense on its own channels. why should Twitter agree to be a propaganda platform for China?

Twitter can't police every post, but if a post gets a million views and is a blatant lie about human rights abuses, it's a bit silly to argue they should let that slide because it's an 'important statement from a foreign government'.

If the NYT tweets about this statement, and attaches the proper context based on generally accepted standards of proper journalism, it wouldn't have been an issue. You can talk about things without being a propaganda piece. Hitler advertising in the NYT in the WW2 shouldn't be accepted. Journalists writing about something Hitler said and putting it in the context (i.e., mentioning the human rights abuses), is fine. Apologies for the obligatory nazi reference but it makes explaining the principe so much easier, even if China isn't as bad.