Anyone can fork the repository and make releases if they wish. Anyone can convince witnesses to run those releases. The only restriction in the code means that you have to run the code on this chain, produced by the witnesses, who are voted in by the stakeholders.
Emphasis on control of the repo is irrelevant — Steemit has no privileged position in the scheme.
Where "this chain" is the chain where the rules are decided by Steemit Inc. Any violation of those rules would mean you end up on a different chain, disallowed by Steemit Inc.
The consensus rules are applied by the witnesses who produce blocks, not us.