I don’t think drug dealers are very thoughtful. Whatever makes quick money is fine. And they probably don’t even know what’s in their drugs.
https://www.wsj.com/articles/fentanyl-crisis-fed-by-flow-of-...
The chemicals are still making it to Mexico from China, that is still easy to track:
https://www.cnn.com/videos/world/2023/03/08/fentanyl-pipelin...
https://www.texasstandard.org/stories/fentanly-drug-path-chi...
Etc...
TLDR; China is still the source for the vast majority of fentanyl in the states (a lot of it is seized in Mexico in containers that come from china, though they get around this by just shipping a lot of it), and no one (including China) is really claiming otherwise since it is just too obvious ATM.