I have to assume that King is white. I do not know why the story does not mention his race.
The term “cracker” refers to white slave owners, not blacks.
The news report video at the link, clearly shows that John King is a white man.
I think the term “cracker” was originally a derogatory term for lower-class whites, but now used by black Americans as a term for whites in general. Probably there were not many slave owners among the pre-1865 crackers.