Censorship does not require an act of Congress. Private entities are perfectly capable of censorship. How else could the term "self-censorship" have any semantic value?
"I won't use the word retard in my project hosted on github because github's TOS prevents it" is not self-censorship in any meaningful sense of the word. (Just as "Github doesn't let me spam all these users" is not Github censoring people.) Nothing is stopping you from hosting the repo in a wide number of other places.
"I won't insult my employer in this or that nation, because I risk a jail term" is self censorship.
Regardless, the term "self-censorship" is not germane to or inherent in only citizen-government interactions.