He's a stupid little 9-year-old kid, what can you expect? He might've just thought that the info ain't essential for anyone. He doesn't know of Robin and Law's goal of finding out what happened in the void century iirc. Plus, the questions he asked then in Zou off-screen have to be somehow tied to the story and this is a perfect way.
Nah. Momo knows full well Robin is searching for the history. Luffy confronted him directly about the ponegliffs saying that Robin was searching for them and the true history.
Either way, Yamao knows void century secrets, some of which we're not finding out until Raftel.
If Oden wrote about the D clan, and the dawn, then it's possible he also wrote about Joyboy and his nakama, especially considering the entire reason for opening Wano's borders (his greatest aim), is to prepare for Joyboy's return. It makes far more sense for this to be new info to Momo, rather than having just known all this incredibly important stuff all along and not told anyone. But as I said, for Yamao's case on the crew, it doesn't matter either way for this point. Her knowledge of the D clan already ruins her chances.