I subscribe to a descriptive rather then a prescriptive view of language. It is "reasonable" to use a term if the usage conforms to how the term is understood by the people using it.

In Sowell's case he uses the term to make a point. The white connotations of "redneck" is apparent as it conveys his thesis that blacks in America got their culture from rural whites.

My point is not that "redneck" is not a racial slur because it could hypothetically be used against non-whites. I guess it can but I have never heard it in practice, save for Sowell's rhetorical use. My point is rather that a white person can use it against another white person, in a derogatory manner, without invoking statements about racial inferiority. In the majority of cases that I've heard someone use the term "redneck" it is one white person insulting another white person's behaviour or background. Clearly this implies something like classism rather than racism.

Speaking of Thomas Sowell, I think he is mistaken regarding black culture. James B. Stewart has formulated a good critique of his stance, if you are interested.