It implies there’s only a single element standing between mediocrity and greatness, when in fact, there are usually dozens, sometimes hundreds.
The word “secret” suggests that the person employing the phrase knows something you don’t, and that this knowledge is hidden, rare, and difficult to uncover.
This phrase glosses over the idea that “greatness” is a matter of personal taste, timing, context, and a plethora of other subjective criteria.
His post title was actually: "The Secret Is… There Is No Secret."
Leo is actually quoting Randy. It's the core message of the post.