Earlier quoted context omitted.
I don’t think it’s unfair to call out the first sentence of the argument, if you’re trying to persuade with several paragraphs of content and the very first sentence has a gross error, it transcends pedantry in my opinion. Rather, I think it makes the original argument look irrational if they have an extreme exaggeration if they have to use that to make their point.
It's a typo. There isn't some kind of grave epistemic problem with the first sentence where everything that follows is predicated on a falsehood. We don't discard the contents of books just because the publication has a typo.
So yes, I think there is a “grave epistemic problem” with the first sentence that hurts the argument and its credibility as a whole, without having to get into the details of the argument itself.
That you called the first replier “stupid” or even their argument “stupid” is wholly unjustified.