Remember the old domino theory, the one that said communism would spread from China right to Australia unless checked, and therefore let's bomb Vietnam now? Welcome to its new variant. This one says that currency collapse will spread from Japan to the mainland right down to Hong Kong, and therefore place your bets against the HK dollar now.
I won't speak for the Japanese yen, but this seems to me to be another case of falling dominoes that stop at one.
It's true that if the whole world, including the population of the mainland, loses its confidence in the yuan then this currency is doomed. Such a loss of confidence could be irrational and still it would happen.
But irrationality of this sort is rare so let's deal with the rational forces on the value of the currency.
In the first place, the mainland has a closed capital account. This means that you can freely exchange the currency for trade but you need permission to exchange it for investment. It doesn't stop the forex dealers from ganging up on the yuan but it certainly makes it more difficult.
Then you have inflation, or rather, in the mainland, you don't have any. The figures show prices falling or holding steady, not rising. Studies admittedly show that inflation differences with other countries have only a weak influence on currency movements outside of periods of hyperinflation.