Actually, I'd say that's probably completely and totally incorrect. The southeast is a hotbed of American talent (particularly in North Carolina, Virginia and Georgia) and has some of the best junior clubs and university teams in the entire country - hell, the University of North Carolina women's team alone is probably the greatest women's football club in the history of the world, and Duke, UNC, Virginia, Furman and Wake Forest on the men's side are perennial powerhouses built on their local talent. Much of what makes college football attractive in the south (the regional pride, the tribalism, etc) also makes soccer attractive. There's a huge cross-polination between the fan groups.
The fact that there is a not an MLS club in the southeast (Atlanta, Charlotte or Raleigh for example) has much more to do with MLS regional politics and the fact that professional sports have never truly flourished in the southeast more than anything else. The Carolina Railhawks have a 10,000 seat stadium in the Raleigh area, and the Atlanta Silverbacks have an entire complex to themselves as well - and those teams play in the second division. Make one of them an MLS team and they'd be packing in huge crowds.
The illusion that football / soccer isn't widely popular in the most sports crazy portion of the United States is just false.