I'd disagree with you that speaking with a southern twang doesn't disqualify a person. I came out of deep East Texas and I have a bit of a drawl in my voice that comes out when I'm tired or stressed but for the most part, I intentionally speak as closely to SAE as possible. Because people do disqualify me and I think you may have judged your friend too. You say you associate his accent with people considered non-intellectuals? That's exactly what LordKano is saying he tries to avoid by speaking SAE in the business realm. Because he believes someone may make the judgement call that he is unintelligent because of the way he speaks, just like you associate speaking like a redneck to unintellicence.

I don't think this is about dialects being associated with any sort of people group. I think it's about having a standard way of speaking in a professional environment. And deviating from that may cause some people to have a negative bias against you.