I still think that the optimal amount of sleep really depends on the person and you can't paint everything with a broad stroke or advise everybody to "have 8.5 hours of sleep". I've found myself the most concentrated and productive the next day when I don't sleep too much, e.g. when I just sleep for 6 to 7 hours. When I sleep a lot I usually just feel too relaxed/hyperactive for some reason and can't properly direct my attention at tasks. I think there should be people with similar experiences out there.

Those statements might be good for raising more public awareness so that they're not chronically extremely underslept, e.g. consecutively for 4-5 hours a day. But to only advocate sleeping a lot is too partial IMHO.