Sadly, this really just isn't true. You're thinking of white people as matured adults who've already been spoon-fed whatever liberal or conservative propaganda that first spoke to them, and been led down that path for some time. But, there are enormous sums of people out there who either aren't really that invested in political thinking, or haven't really been guided to a more radicalized point of view. And, just like the former group, the latter group often views the word "cracker" as a slur equal to any other, not having the historical context to weigh the use of that word against another. So, when you use that word, they're going to take it very seriously, regardless of how sarcastic you're being, and regardless of how much a word like that means in comparison to a slur used against a marginalized group.

I know this because many people start out from this position - all slurs are equal, everyone deserves equal treatment, etc. It may not be how the US behaves, but it is what people are taught in grade school (usually).

Well, I said "sadly", but to be honest I don't view this reality as something sad. I think it's actually quite a good thing not to simply write off anyone who will react in such a way, because like literally everyone here on Hexbear or anyone else, context is not naturally inherited - it needs to be taught, and with patience (and the interest to do so) you can often open people up to a greater, truer understanding of things. If you just call them a cracker and tell them to fuck off, then you're effectively further barricading the door to leftism against them. That doesn't mean tolerating any kind of hateful words or misinformation they might be spouting, it just means not giving up on everyone who doesn't super love it when you compare them to mayo or whatever.