To be a pedant, I can't imagine the coiner of this phrase or really anyone who has ever thought about it uses the term in the sense that something or someone became successful because of actions taken over a proverbial overnight. The examples he mentioned are things which became popular abruptly, which seems to embody the spirit of the phrase just right.
To take the point to the extreme, absolutely nothing could be considered an overnight success if we use the term in the sense the author implies because noone is wildly successful at something they attempt for the first time if they have no apriori knowledge of how to do it from previous attempts or study.
To take the point to the extreme, absolutely nothing could be considered an overnight success if we use the term in the sense the author implies because noone is wildly successful at something they attempt for the first time if they have no apriori knowledge of how to do it from previous attempts or study.