Cryptocurrencies aren’t banned per se, just exchanges. But do bitcoins have value in China if you can’t exchange them? That’s the main question and the reason why cryptocurrencies are crashing.
This is most likely one persons interpretation of china's law that crypto currencies are not allowed to be used as a currency. They're only to be used as a commodity meaning you cannot purchase items or services with them like you can in other countries. Lots of people took this as you cannot purchase currencies with money.
Some sites were saying that some chinese exchanges were already stopping the purchasing of cryptos, but not the sales. Which makes no sense, if they were going to ban one they would ban both.