I think the most accurate way to point out the most obvious flaw in the reasoning is:

You haven't heard about XYZ in fantasy world yyy, doesn't mean XYZ doesn't exist in that world. If you take star wars, before episode 1, we hadn't heard of jar jar Binks but he existed in that world and had (surprisingly) big influence. Proving that something doesn't exist is literally impossible. Thinking that something doesn't exist because you weren't exposed to any evidence for it, is flawed thinking. Especially in a fictional stories... especially in a fictional story that wants to talk about a specific issue.