I don’t recall whether Ivan Allen coined the phrase, but it does go back to the 1960s when Atlanta was trying to set itself apart from other Southern cities, particularly “Bombingham”. It was probably more a result of Martin Luther King not wanting to crap where he ate than any real liberal tendencies, although Bob Woodruff did pull strings from his plantation in south Georgia where the black help was well behaved.
There is a clip of Allen responding to a reporters question with that answer that plays at the civil rights museum at Olympic park.....right across from the Coca Cola exhibit. If you haven’t been it is worthwhile....except gor the top floor which is about Palestinians and other communist nonsense