When did he change his mind on this?
I can't find the exact thread right now, but RMS asked someone to use gcc instead of clang when proposed for code completion in emacs... and then refused to allow the necessary changes be made to gcc to allow code completion because evil corporations might use it to extract the AST from gcc and therefore bypass the GPL.
How do you know that? Especially so, given that tons of "social coding" happened long before github was a thing?
Or the fact that there are more software developers in the world today than there were when github was created wouldn't have happened without github?
Or is it that because sourceforge wasn't a quasi-monopoly, therefore everything that was not on sourceforge didn't exist?
Or what exactly is your point?