If you ask people in 49 states who General Sherman was, it wouldn't surprise me at all that lots and lots of them don't know.
They asked 522 Georgians.
I'm sure there are Americans who don't know who Henry Ford was, but they don't live in Michigan.
As for Civil War Battles, I'm sure a lot of Americans would get Gettysburg and draw a blank after that. Of course, anyone over 50 has probably seen "Gone with the Wind", and ought to be able to come up with Atlanta. In the state of Georgia, though, I'm guessing that even people who have never watched the movie could come up with Atlanta. If not, maybe they might have heard of one of the 28 other battles Wikipedia lists as having been fought inside the state of Georgia.
Either the questions were worded in an incredibly deceptive manner, or people deliberately left them blank, or there were very, very, few people who were represented in the statistics cited in the article.