That would require a lot of extra mortality to be hidden, which is possible. But it would also require that there are not a lot of hidden cases. We've seen plenty of mild cases in west. With our testing we have not missed a lot of cases, hopefully. The question is how many cases have the Chinese missed.
In short the mortality could be anywhere from a little over 2% (their announced rate), higher, or much lower. We don't have enough information to know yet.
I would think that the 15% would be for untreated people. You could probably stay home and treat yourself better than being warehoused and ignored in a hospital.