I would say this is about status. People filter what they say to high-status individuals, but don't bother filtering what they say to low-status individuals.

Nerd culture is traditionally low-status in context of the whole society, and meritocratic inside. That means that nerds are used to hearing non-filtered things from outsiders, and don't have strong reasons to learn filtering when speaking with insiders. Also, it is more complicated for aspies to understand when and why exactly should the filters be used, so it is easier to have a norm for not having filters.

(And I suspect that people most complaining about the lack of filters would be often those who want to be treated as high-status by the nerd community, without having the necessary skills and achievements.)