China doesn't need to 100% enforce the law to effectively make something illegal. Even if they catch and prosecute 1%, their justice system is dangerous enough that most people won't take the risk.

For crypto as currency, this is effectively a death sentence. If most people are afraid to hold or trade it, you can't use it locally.

It would only be useful to people hoping to exfiltrate value and then leave the country, which is not a majority of people.