Hostility may have been too strong of a choice of word. It's less the arguments or sentiments of anti-car advocates but more the language used online.

For example, r/fuckcars is massive. A large subreddit might not be considered mainstream, but it does have massive reach. A lot of people see this as one of their first exposures to car-free communities and do not feel welcomed by it.

I think it's somewhat of an optics issue, but it is very hard to find walkability communities online that aren't using terms like "car-brain". I know that these terms are targeted at a certain type of car owner, but without being familiar with the car-free community this is not always clear and can be a turn-off to car-owners who may be curious about learning more.