This phenomenon, of great scholars not being aware of things that most people reading the Seforim blog learned years ago, should not surprise us. The traditional yeshiva curriculum is very narrow, and you can spend your life in a yeshiva and unless motivated to expand your horizons, will have no knowledge of entire areas of Jewish thought and history. A good example is seen in this announcement by Agudas ha-Rabbonim, which appeared in Ha-Pardes, November 1975.
Yet the beautiful thought which the learned rabbis assume was stated by Hazal was actually stated by Ahad ha-Am, and is perhaps his most famous saying. However, for one whose only Jewish knowledge comes from the yeshiva, this information would be unknown, and it is easy to see how such a statement (“more than the Jews have kept the Sabbath the Sabbath has kept the Jews”) could “infiltrate” this closed world and become just another ma’amar hazal. It reminds me of how when I was a kid and my friends and I went to Boro Park for Shabbatons we would have been able to hum niggunim which came from popular songs and commercials, and our hosts wouldn’t have known a thing. At the Rutgers Chabad house in the 1980’s they even had a niggun to the tune of the theme song for Bumble Bee tuna. For those too young to remember it, see it here.
Of course, Ahad ha-Am’s statement is sound Jewish doctrine, as should be expected from one who had a hasidic upbringing (he was born in Skvira). I don’t even think that the saying was original to him. Rather, he was repeating a hasidic idea that he heard in his youth.