Carnist, I believe, is a term used by vegans to refer to those who support ideologically that it is acceptable to use animals for food, rather than someone who eats meat. There are people who do not eat meat, for religious or cultural reasons, and those are generally referred to a vegans, in the same way there are those who eat meat for cultural reasons (my father did it, his father did it, I've never really thought about it) rather than supporting some ideology. For vegans to label meat eaters (more accurately omnivores) as carnist, their ideological/political opponents is, well, not really offensive, but it does rankle.
As social psychologists say, there is no in-group cohesion without out-group hostility, but it is a shame for vegans (for which I have some sympathy) to isolate themselves in this manner, particularly if they want to change the world for the better.
People choose all sorts of inaccurate terms for themselves, your reasoning doesn't seem particularly sound either.