That principle doesn't work online. In general you can say stuff to people in person that's insulting and abusive, and then just give a wry smile and everybody laughs. You can't do that online. The guideline has it exactly wrong.

The problem is that what you say online about someone is frozen forever, and takes on more significance than either the sender or the receiver actually meant. In person, I call you an asshole, you're impressed that I said it to your face, we both laugh and move on. On the web I call you an asshole, you call me an asshole, it escalates, shows up in google searches for both our names, people who aren't quite up to speed on things (recruiters) take it more seriously than it should be... bad stuff happens.