Retreating to a dictionary definition doesn't argue against the point of the GP comment, namely: a white guy being called a cracker doesn't resonate with the kinds of cultural practices of oppression that that using racial slurs against a minority carries
If you're interested in arguing against that, do so. Posting definitions of words from dictionaries is the height of pedantic laziness in non sequitur form (unless the argument is actually about what the dictionary definition of a word is, of course).
Subsets of academia focused on "critical race theory" might try to redefine "racism" to suite their specific agenda, but I'm content with the dictionary definition:
"the belief that all members of each race possess characteristics or abilities specific to that race, especially so as to distinguish it as inferior or superior to another race or races."