mmmm I'm skeptical of a singular study in which a certain tribe of people are asked to modulate a sound to an octave within their singing range disproving octave equivalence being hardwired.

Wanna know why? Our aural circuits have frequency binning, much like an FFT, and they "ring" at certain frequencies, and an octave is either 1/2 or 2x a given frequency/"note". Closer examination of this system under stimulus is what I would consider a gold standard in this field of research. The form of research undertaken in this study suggests many things, and the researchers have chosen a particular conclusion and it's not necessarily indicated by even their own data.

https://www.britannica.com/science/ear/Analysis-of-sound-by-...

Given the current propensity for thinking that frequency detection is pre-cortex (lower level than speech decoding) and available rather early on in the evolutionary chain, I'm going to be interested in physical studies, not social science.