I swear the “fuck cars” crew are completely deluded from reality.
I see people say what you're saying (bus vs car road damage elasticity) in "fuck cars" communities, I don't really see why you've decided to attack them collectively. But it's a pop-community, they're going to be wrong every now and then either way, please give them some slack. Their purpose is to make an average person aware of car dependency and that it's generally a negative thing, so that actual urban planners with technical knowledge have an easier time arguing for and implementing realistic solutions, and they'll take into account the variables you bring up. Think of "fuck cars" like a form of lobbying except it's done by common people with good intentions - similar to how Japanese coops lobbied for better food safety standards decades ago - rather than wealthy corporations.