Although I get what the translator is saying, saying "feeding milk will kill your cat" is a drastic oversimplification, I think? (Edit: I see the comment that the scanlators also know this but stuff happened XD) Leaving aside kittens, (for which, like most mammals, things go very wrong when fed the wrong species' milk—and I feel terribly sorry for the experience of the poster before me).
Many adult cats are lactose-intolerant (like adult mammals of most species), but just as with lactose-intolerant humans, that's not going to be fatal, I suppose unless you ignore all the signs of digestive distress and keep feeding them milk. And for those who aren't intolerant, it's horrible junk food and shouldn't be a staple, but it's hardly a poison.
All of which is to say, "don't feed cats milk" is pretty decent rule-of-thumb advice (and giving it to a stranger's cat is irresponsible; the gift of potential diarrhea is not a kind one)—but also don't assume that someone's a horrible person who's going to be the death of cat the moment you see them giving them a saucer of milk.