You are right I can only try to infer information from them that’s not provided. I would infer there is a distribution of when people get up and the 8am classes have the largest set of people getting up nearest to the class start time. Some of the people get up very early and the other end is people up so late they miss the class. The later the class the more people are awake so the better the performance. I’m saying the data they do provide does not refute my take on it.
But this study is asserting that the earliest 8am class, of a system that runs 8am to 4pm, has the worst performance.
It can’t say that a 9am class in a system of 9am to 5pm would have better performance at 9am, and it certainly can’t say that the system as a whole would have better performance. Maybe 5pm is too late and affects child care or dinner or night jobs and now students are distracted all over again.
There is way more research than just this study that links early starts to poorer educational performance.
Your speculations have nothing to do with anything in the study, depend on facts that are not supported by any research, and are contradicted by other studies on this topic.