I think the point being made is that "cracker" does not apply to ALL white people in the way that the N-word is applied by its users to ALL black people. So, while "cracker" may certainly be an insult, it's not insulting on the same *basis*.
We have a similarly nasty word here in Britain, a relatively recent coinage - "gammon". It means, essentially, a stupid and narrow-minded white conservative.
Race is *part* of it but not the *whole* of it - it's only applied to a subset of white people, and their whiteness is not even the main characteristic that the term is concerned with.