Don't know what to tell you here. I first heard this from a professor in college that was from Guangzhou I later heard this from friends from Guangzhou and from another friend form Beijing, my first sifu from Hebei, and then my wife from Beijing and my Sifu form Hong Kong and none of them knew each other. I have not asked my Sanda sifu about this however, the next time I see him I will have to ask him what he thinks.
If I can base this on the comparison my wife makes to my temperament (which she says is the much like a Northerner) yes Southerners are quick to anger but they first argue and that can go on for a while or it can escalate to a fight. Where Northerners are not so quick to anger, they kind of skip that whole argument (or shorten it considerably) phase and then go straight to fighting and then it is all over no more anger.
The Professor I had in College also went to University in Beijing and, as she put it, she was rather surprised one day when she got into, what she thought was, an argument with a fellow student (who was from Beijing) and the Beijingren did not argue more than a couple seconds and then went straight to trying to punch her in the head.