Fentanyl is synthesized in China or Mexico (with precursors from China) extremely cheaply and without the need to maintain large plantations. It can then be imported to the US much more easily than heroin (extremely potent by weight and doesn't smell as much) where it's then diluted/mixed with other drugs. This is easily provable, there's tons and tons and tons of evidence/documentation of all of this.
I don't understand the original posters point either though, because providing addicts with clean drugs and access to treatment is the only real way to curb the current fentanyl epidemic.