Ignoring methodology concerns for a moment, there have been times in the past where people have found statistical regularities like this in the stock market. Sometimes people have made money on these discoveries. If not someone else will as soon as the researcher publishes. But making money on them gets rid of them, and given that this is now publicly available information those regularities certainly no longer exist, if they ever existed in the first place.
I'm (and economists) aren't saying that there are never $10 bills on the sidewalk, just that they don't stay there for long after someone notices them. Stock prices went down on the weekends for decades before someone noticed. But once it was noticed it stopped pretty quickly.
I'm more interested in the persistent market inefficiencies; growth vs. value, January effect, low P/E, all interesting examples that haven't been arbitraged away.
I am just trying to double check to make sure I understood so bear with me. Are you telling me that stock prices systematically went down for decades and no one took this into account to make money? there is hope after all...
Most of the time, the economist is right about that: the vast majority of ten-dollar-bill-appearing things found on sidewalks are obviously not real ten-dollar bills once you pick them up. They're advertisements for nightclubs or get-rich-quick schemes and whatnot.
EDIT: Ok, I couldn't resist the xkcd reference: http://xkcd.com/882/