The statement "Of the American dialects, the Southern dialect is the closest to British English" is false.
Southern American English or Southern U.S. English is a regional dialect or group of dialects of American English spoken throughout the Southern United States, though increasingly in more rural areas and mainly by White Southerners.
The major native dialects of English tend to be divided by linguists into three general categories: the British Isles dialects, those of North America, and those of Australasia.