Fair enough. I'll bow to your greater knowledge... but so should a lot of others. If it is a myth (and I'm not saying it is or isn't) it's a well established myth.
There are a lot of well established myths about Native-Americans, especially in regards to Mesoamerica. This is just one of them, but I could make examples of many others.
I've just had a thought which runs counter to my earlier views on this. That is, from my understanding most of the conflict in the New World (New World vs New World) was ritualistic with the soul aim of capturing the enemy for sacrifice.
And this would be another. For one, not even Mesoamericans waged wars for the primary aim of capturing sacrificial victims, let alone the New World entirely. This started out as some weird claim to explain why the Aztecs lost and because people can't differentiate between Mesoamericans (who in the words of one conquistador could be as different from each other as the Spanish from the Turks) it's gotten to be applied to all of them. War was pretty much waged for the same reasons everyone else did it, pragmatism, opportunism, and the occasional conflict over honor. Capturing enemy warriors to sacrifice was considered a bonus really, and even then it wasn't the only fate for captives even among the Aztecs who were by far the most prolific people at that sort of thing. Generally captive enemy warriors were kept as slaves, among the Maya preferably only those of high rank were sacrificed as it was an honorable way to go compared to being enslaved, though even then it would still be common to use noble prisoners as hostages or puppets like in the old world.
As for the use of gunpowder, grenades and signal bombs are very likely uses as far as warfare goes. Don't need metal to make a grenade, they could easily just use clay jars or even bags filled with gunpowder and rock shards. And as previously mentioned, using it as blasting powder for construction would be feasible as well.