There is some other reason they're so strongly against him than "that" right?
...right?
Nah. He's previously claimed to have been the victim of racist abuse at Roker Park - but such a game on the date/year he alleged never took place. Their fans (arguably rightly) believe he's fabricated something for his own benefit, presumably choosing a northern club to portray a stereotype.
This explains it all: https://rokerreport.sbnation.com/2014/7/13/5895103/sol-campbell-sunderland-race-allegations-roker-park