I don't think that's true. Explosives, which are easy to make, require primary explosives and those are not trivial to produce (or stupidly dangerous to handle) and are too heavy for drone delivery. Overall, in most countries, the required chemicals are tightly controlled and you will trip wires, when you try to get your hands on concentrated reagents.

An IED is relatively easy to manufacture in regions where you readily can get your hands on (second hand) military primary charges, shells or grenades. Then you just need to mix half a ton of cheap nitrate-based explosive, add an artillery shell and some wires, and you got a simple bomb with a yield that will wipe out a convoy of armored vehicles. But in any case, you won't fly out such a bomb with a drone, since the weight to boom ratio is pretty bad (you would just fly out the pre-made "primary load").

To me at least, the drone thing only makes sense, if you got abundant access to grenades or shells.