I've changed my opinion since my first post. "Long since" doesn't need a reference point if it means "a long time ago". Roman and emsr2d2 are both right. "A long time ago" obviously has 'now' as a reference point. But I'd say that "long since" is idiomatic. "Since" is unusual in this construction because it doesn't have a reference point in the past (except for "a long time ago").
Maybe...maybe...as a foreigner, sometimes he or she only needs to grasp the basic meaning.
After all, the English ethic masterpieces are really not simple for foreigners.
PS: The original sentence is from selected English ethic masterpieces(written by English or American ethicist and published by Peking University_____haha,not bad university).
Thank you! Thanks all!