Did you read the article? He points out “It’s possible that Night Shift does something, but the biggest study I could find of Night Shift mode (still a pretty small study) found little effect on sleep, so if there’s an effect, it must be tiny.” He links the exact type of observational study you asked for

Regardless the maximum possible effect will be constrained by the biology of the cells responsible for responding to blue light. Maybe knowledge of the biology is incomplete or flawed but to not use it to inform what’s possible seems foolish.

So what if it’s a placebo effect? Well some people are spending money and time investing in blue light filtering glasses and other solutions. It’s potentially snake oil and it could keep them from pursuing better solutions that would actually help them sleep