Your point about "worse jokes [...] on 4chan" is important. Wishing cancer onto someone is almost embarrassingly mild on 4chan. The idea that someone would take offence to that ancient insult is laughable. Outside of 4chan and without that context, it is actually a pretty harsh thing to say. And even if I personally see and understand the humor, I would definitely disallow that kind of language in any workplace I managed.

I'm just pointing out that Facebook is setting the limits of its platform. You suggest that if a human saw your joke, they would recognize it as such and allow it. Perhaps they wouldn't. Just because something is meant as a joke doesn't mean it is appropriate to the circumstances. There are things that are said clearly in jest that are inappropriate not merely because they are misunderstood.