So little lock-in?
Git itself is simple, but is their .md file renderer open? Are their workflow runners available anywhere to re-use? The API also has gaps (esp if you wanted to migrate your PRs or Issues over), though I don't remember what was the thing I couldn't achieve with it.
But I guess you are referring to indirect, incidental domination like "social" lock-in (I always smile at job application forms asking for a GitHub link — I've got thousands of free software commits in their own, dedicated software forges, but very few things on GitHub directly, or at least, in mirrors not tied to my GH account), "marketing" lock-in (I've heard many junior and not-so-junior engineers refer to GitHub as "Git"), etc...