I don't think it's that simple. GitHub themselves say [1]:

> You're under no obligation to choose a license. However, without a license, the default copyright laws apply, meaning that you retain all rights to your source code and no one may reproduce, distribute, or create derivative works from your work.

Yes, it does note immediately below that "if you publish your source code in a public repository on GitHub, according to the Terms of Service, other users of GitHub.com have the right to view and fork your repository". But that doesn't mean it's open source. You probably can't/shouldn't create derivative works with your forks. And you definitely wouldn't be allowed to just use this code in a commercial product, for example.

[1]: https://docs.github.com/en/repositories/managing-your-reposi...