We aren't talking about the users of this board, we are talking about the average American who went through the public education system. 99.9% of them have heard of George Washington Carver, but not of Onsager and Landau.
Your original interpretation was right. Perhaps I underestimated the number of physicists hanging out here. (I hope so! It would be cool to think people here know who these guys are.) In any case, the original point remains: even on Hacker News, Marie Curie is far better-known than Lev Landau, and it's not because she was the greater scientist.
