I enjoy your explanation, but I disagree.
What you’re suggesting would mean that the functionality of the hardware (volume buttons) would change.
The Volume Up functionality is relative to the “Top” of the iPhone, which is indicated by the side of the phone opposite the home button.
Because it is a physical device, reversing the functionality of two hardware buttons based on the gyroscope is too brittle of an experience.
The volume up and volume down should work the same at any angle.