Phd's Logic Loses Me

Professor Mankiw of Harvard has a post, the logic of which escapes me.  To oversimplify, the question is whether education is the key factor in the rise of the "1 percent".  Professor Krugman argues it isn't, Mankiw in his post argues it is.  But his argument is weird: he says both he and Krugman are in the 1 percent because they both have doctorates (or at least that's how I understand it). If their education had stopped at a high school diploma they wouldn't be in the 1 percent. 

Seems to me the comparison doesn't work.  If education was the or a key factor, one would expect college professors to be in the 1 percent.  They aren't, the examples of Krugman and Mankiw to the contrary.  Depending on the year, the top 1 percent makes between $350000 and $450000, well above the salaries of almost all professors.  (Harvard's average was about half that.)