For a while China refused to prosecute these manufacturers unless they sold to Chinese citizens.
Thus China is DIRECTLY trying to get revenge from the 100 years of shame
First of all, are we talking about manufacturing or trafficking? Because fentanyl is legal medicine, used by hospitals. It isn't illegal to manufacture, use, or export when complying with controlled substance laws. What's there to prosecute?
There were some shady outfits that sold to individuals online and those should have been shut down if it was so easy to establish a link, but usually it goes through many hands long after it exits China. Like guns and "Made in China" goods, it's a supply-chain phenomenon that the source manufacturer happens to be in China. Doesn't mean China controls the whole trade. Obviously if the sale is in China for illicit uses there is a lot more big brother tracking and when found out the draconian drug regime there applies. You're reading non-existent things into sensationalist news stories.
Last year, China went along with the DEA to ban the manufacturing of some synthetic variants in illicit use, so they're helping if anything. They didn't have to even talk to the DEA when it's the DEA's job to crack down on drugs on the home turf where the real problem is.