Adding China would require implementing a unique government form that's not transferrable to any other country in the game. Hence why it wouldn't work. If you want to know more, then I suggest you read this thread where China experts discuss what would need to be in order to accurately model medieval China in CK2. https://forum.paradoxplaza.com/foru...nics-would-need-to-be-added-for-china.859962/
Your own link seems to disprove what you say. The first post (with 17 upvotes and no downvotes) indicates that china could be represented quite easily and isn't beyond the scope of previous DLCs like HL or SoI. Also china is quite a large region and wasn't united for much of the CK2 period(5 dynasties 10 kingdoms anyone?) so it wouldn't just be a single country using the government type even if we assume vassals are removed.
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