@givemersspls Not in my opinion, not at all. In 99% of shounen everybody always speaks the truth, intentionally and unintentionally. In fighting shounen the worst villains always speak the truth to the heroes, be it their plans or powers. Nobody ever lies. Is this what feels correct to you? I'm no doubt a relatively old geezer by MD standards, but even so every year or more I find out I've always been thinking (believing) incorrectly about something, like a physical phenomenon, a detail in a historical event/war, technological thing, etymology of a word, the exact limits in the wording of a law, etc. I find it completely natural young children, still mostly devoid of abstract thinking and believing every word authorities tell them, would be wrong about a whole lot of things, even ridiculously obvious ones, never really questioning them. In fact in the comment sections of sites like MD or Youtube you encounter pretty much every day people who are blatantly incorrect about details, unfortunately myself included every now and then. That's life.
@givemersspls I admit it was highly convenient for this scene, having that single misconception solve the whole situation. Considering Himeno's personality, she ought to know it's incorrect. She shows pretty mature behavior (in the bad sense) by not denying it since Ouji's incorrect belief suits her purposes. The cousin, however, simply believes Ouji due to being an easily influenced kid and, furthermore, it was someone she likes who said it. In this sense I think it had these two sides: While it was a too convenient plot device, it does also show how children operate.