The claim isn’t that you’d never do that. The claim is that even when you wouldn’t do it—even when you wouldn’t even bother to look at x_41 from a God’s eye perspective because it provides literally zero predictive power conditioned on x_1 through x_40 - even then, you might find that (as a human limited by human reasoning and human coordination) you get the best results when you use x_41 alone for your decision boundary.
why wouldnt you just use x_41 all of the time if you only get one category—why is this different from what you’d pick at a gods-eye view?
The claim isn’t that you’d never do that. The claim is that even when you wouldn’t do it—even when you wouldn’t even bother to look at x_41 from a God’s eye perspective because it provides literally zero predictive power conditioned on x_1 through x_40 - even then, you might find that (as a human limited by human reasoning and human coordination) you get the best results when you use x_41 alone for your decision boundary.