I think they're more so talking about measuring attenuation that apple applies for the "spatial audio" effect (after apple does all of the fancy IMU tracking for you), by using a known amplitude of signal in, and the ability to programmatically monitor the signal out after the effect, you can reverse engineer a crude estimated angle out of the delta between the two.

I don't think that's how this app works though, after installing it I got a permission prompt for motion tracking.

Looks like there is an API for this. Here's an example: https://github.com/tukuyo/AirPodsPro-Motion-Sampler/blob/8ac...