Well...

It is plausible enough on the surface, at least for fentanyl. If you see carfentanyl, sufentanyl, or the like in the wild, it's a god-damned miracle. Fentanyl is easy to work with; don't be a do[e fiend, stay on naltrexone. Its more-potent analogues are "accidentally kill a village on the other side of the world"-tier.

However, while China and SE Asia in general are known for designer drug manufacture in the white market (making fentanyl a "people are dying because narcissistic overlords hate harmless psychedelics" problem), there's been no actual evidence that china holds a role, and in fact, they've moved to control the substance. Nor is there any reason that it would require any particularly sophisticated actor; fent is an astoundingly easy synth if you have some 1,5-pentan-3-one lying around, with the main other ingredient being vinegar and a spot of benzene, and probably doable if you have intimate knowledge of piperidine substitution... from black pepper, which is not exactly heavily controlled.

In other words, any sufficiently-determined gradeschooler should be able to whip up a fuckton using nothing but empty glass jelly jars.

Because of this, geography is a much more useful tool in trying to figure out "who dat?" Since all the fentanyl issues seem to be in two places, that leaves two major candidates...

...Russia, and the Netherlands.

China probably has little to nothing to do with it, since humping fent overland from china to poland just to throw it in with the eurodope is... unlikely.

But, that's just a guess; I haven't really studied the issue in-depth.