They are not extremes. It is different at different latitudes, be quite a bit. However the extremes prove the point.
we’ve arranged our society around 8 am.
No we haven't, sorry. There are plenty of people who would say that is 6. Plenty of people who would say that is 9.
Again, go look at what happens at different latitudes and times of year. You will see that this argument that there is a morning by the sun that is universal and stays the same all year round fails.
Shifting the arbitrary number does not change that. Their argument is complete nonsense.