First, it was a joke in response to a joke. That much should be obvious to anyone. So it has the accuracy inherent in a joke.

And second, since it was about large numbers of people, it was obviously a claim about the vast majorities and not a statement indicating, for example, that there were zero exceptions.

The offense is more in the responses than in the original comment. The comment first got many upvotes because it’s obviously a joke. But then some people with poor reading comprehension invented offense and responded like it was obvious, then everyone started acting like chickens with their heads cut off. The people who could read and think for themselves didn’t respond because it’s obviously just a stupid joke.