And then they use that cash to buy a house. From the article:

Indirectly then, the capital controls in China are enriching the drug cartel leaders as they figured out this alternate method that essentially moves the proceeds of their drug sales through China.

And to go further, the reason the drug cartels are profiting in the first place is because with these drugs being illegal they receive a high margin selling them due to lack of competitors in the space.

So the bottom line is that regulations on certain drugs in the U.S. led to drug cartels flush with cash but with the situation that the cash is located where it was hard to move. And then capital controls (regulations) in China led to a solution that ultimately let those drug cartels have access to that cash locally.