Is this actually true? It makes absolutely no sense at all, unless the goal of whoever is doing it actually is to kill people.
Cutting heroin or other opiates with Fentanyl can make sense (from a dealer's point of view), because it is extremely potent but not much more expensive than the desired drug -- so you can mix a tiny amount of fentanyl with loads of fillers and then cut that into the actual drug, and your buyer will probably never know. Until you screw up and put a tiny bit too much fentanyl in and then lots of your customers die. But this model, however fucked up, makes sense.
Cutting things like weed with fentanyl truly makes no sense to me at all except as a fucked up way to murder people.