I think part of his point was that expecting everyone to have a thick skin is a small step away from requiring everyone to have that thick skin, and to not reacting appropriately to people who direct flames against those supposedly thick skins. “Thick skin” is often used but poorly defined when talking about online communities, and I think the end of the article does a good job of deconstructing the notion by explaining more precisely what is a good standard for reacting to abuse.

Another point to make is that this is a stronger expectation than simply: don't be an asshole, and that acceptance criteria that don't match the emotional reality of most people (for example, if “thick skin” means you're not allowed to feel bad after being treated harshly) will ghettoise a community.