The dirty little secret among academic historians, especially the famous ones like McCullough, is that they get huge research grants and then use the money to hire research assistants to do most of the work for them.
Generally you're right. I've never, ever received a "big" grant--two or three very small ones from Earhart Foundation. At most, all I've ever done is ask my ONE researcher to check citations that I have already made to make sure I got them correct, or to double check things I cited from a secondary source and to locate them in the primary source, for the same reason.
I have no reason to distrust McCullough, however.