Really?
Didn't know that.
Can you name a few of those tribes?
Here's two. I'm trying to capture the links on my phone's browser but the formatting (text and url data ) is difficult.
Pirahã people - Wikipedia, the free encyclopedia
The Cincinnati Quarterly Journal of Science - Google Books
Cincinnati quarterly journal of science - Volumes 1-2 - Page 283 -Google Books Result books.google.com/books?id... Samuel Almond Miller, L.M. Hosea - 1874 - Science
The Toupinambas of Brazil had no religion.
Those studies really don't relate to this issue. They deal with a more comparative idea of organized religion and beliefs which reflect our own views of religious thought. If you looked further into those studies you would find a variety of beliefs dealing with the supernatural which need to be taken on their own rather than as a comparison to western belief.
I understand your perspective but the inference from the alleged "study" that has been copied and pasted multiple times was assigning the Christian concept of "gods" to a culture with no such conception.
Similarly, I would propose that our definition of "supernatural" is a term and a conception that would have been unknown to tribes, isolated from such ideas. It's arrogance and cultural bias to impose the Christian conceptions of gods on a people who have no such conception (or use) for those gods.
Remember that Christian theism carries with it a lot of baggage such as conceptions of a unique triune god(s), a flawed humanity, a savior, heaven and hell, Satan, etc.
There is no reason to expect a culture isolated from such (anti-human) ideas would spontaneously generate such theism.