It's not clear that it is more socially unacceptable to say something like "dumbblack"... the workplace he was at essentially said that to him repeatedly for months, while he had a single rant on a webpage. But if you look at the breakdown of Hacker News comments, it's pretty clear where most people's sympathies lie.
That's like saying that steak is food, but ramen isn't. Clearly, steak is considered a higher-quality food, but that doesn't negate the fact that ramen is food as well.
Due to the historical mistreatment of black people in America, it is more socially unacceptable to say something like "dumbblack"; however, "dumbwhite" is no less accurately described by 'a hatred or intolerance of another race'.