The gourd rattle is found across essentially the whole of lowland South America, among peoples who share no language family, no political history and very little else. It appears in the Amazon basin, along the Orinoco, in the Guianas, in the Gran Chaco and in the eastern lowlands, and it is generally the central instrument of the practitioner wherever it is found. Few material items in the atlas have that kind of distribution.
Explaining it does not require a diffusion story. The bottle gourd is a cultivated plant of very great antiquity in the Americas, the construction is obvious once you have a dried gourd, and a rattle is the most immediately useful noise-making object it is possible to build. A distribution this wide and this old is better read as a shared inheritance than as something that spread from one place.
What the distribution does allow is comparison at a finer grain. The interesting variation is not in the object but in what is put inside it, what is carved on it, whether it is treated as animate, whether it has a personal name, and whether it is buried or destroyed when its owner dies. Those answers differ sharply from people to people, and they are where the ethnography actually lives.