The full joke is usually:"Some people ask me if it is a sin to use the Other Editor, vi. It's true that vi vi vi is the editor of the beast, but if you use a free implementation of vi, it is not a sin, but a penance."
This is just poking fun at the editor war.[1] I don't get the impression that Richard Stallman really cares; he just doesn't like vi for practical reasons, which is why he initially wrote Emacs.
[1] https://en.wikipedia.org/wiki/Editor_war
