If you just want a place to publish your code, you can use GitHub like any git repository, in a decentralized fashion. If you do that, you are not dependent on GitHub, it is just another clone.
But as soon as you start using the features of GitHub that are not git, you become dependent on the platform. AFAIK, you can't easily push and pull bug reports and discussions. If you want to start a community elsewhere, like a competing platform, or maybe a traditional mailing list, and there is already something happening on GitHub, you are creating a split.
You may also not trust GitHub and want to be able to leave at any time. Again, no problem with git, it is just a matter of changing your remote, but everything that is not in the git repository may be lost.