Do you assume that because someone doesn't recite the textbook phrase to describe a photographic phenomenon that they are just hopeless idiots who have no understanding of photography?
If you're imagining a blue sky and it's relationship to a grassy lawn; there is contrast between them. That is, there is a degree of difference; small or large. Adding a dark red filter will darken the tonal rendering of the blue sky, and thus the contrast between the sky and grass will change. Surely you can see how this affects contrast.
Ok... looking forward to your rebuttal.
edit: actually, after rereading the OP I kind of see what you're saying. nevermind!