Contrary to what some people believe...
not everything ever published is available online (yet). There are still millions of old books whose copyright has expired that have not been made available online (partly because no one is willing to scan or retype them all). That doesn't even begin to touch on all the old newspapers, especially ones from over 100 years ago. Hell, it's hard enough to find some articles from 2 weeks ago on a lot of newspaper websites.
He mentioned Shelby Foots's 2,300 page Civil War trilogy, so perhaps he found the quote there. I'd like to see a direct source, though.