If 60% people perform better with light theme and 40% dark theme, average is going to be that light theme is superior. This doesn't make dark mode a bad idea.
All you're saying is that means aren't informative if distributions are bimodal. Which is both totally true and completely missing the point.
This is going to appear nitpicky, but the average preference is some average gray, neither dark nor light. The mode is light. That relates back to the fighter jet thing: if you’re going to build a non-adjustable cockpit, build one that fits (within some tolerance) the most number of pilots, not the average on every dimension.