Obviously, a Harvard Ph.D. is somehow distinguished (or just different) from just an ordinary Ph.D., as the astute and erudite author deliberately uses extra keystrokes to describe his degree in those terms.
Many people who really went to Harvard take pains to avoid saying they went to Harvard. I recognize that the opposite is also true, but the former statement is such a commonly observed phenomenon that Harvard alumni joke about it.
At any rate, the distinguishing factor between a “Harvard Ph.D.” and a regular, old, boring Ph.D. is the extra helping of smug that comes with the diploma.