LOL! It had nothing to do with “public vs private” education in that sense. Public schools rarely teach important facts about American history which has been subsumed within their multicultural social studies curriculum. That being the case, the comment about who freed the slaves has relevance. It came right after an exhortation to register these fleeing from the north blacks in the GOP.
P.S. But since you brought it up, you can see the results for yourself usually each year when they give the results of American history tests on the news. They're are huge numbers who don't even know when the American Revolution or the Civil War happened.
There were some discouraging comments following a post on one of the icanhascheezburger.com affiliate sites. There was a post about an old Revolutionary War soldier who, though wounded severely, ended up dying of old age at 98. Some comments following the article expressed indignation at the fact that this individual had killed three British soldiers and they simply couldn't understand why he'd done that.