I don't know, to me it just seems like a plausible interpretation:

- In GitHub, forks of public repositories are themselves public repositories.

- GitHub repositories can be cloned, which is a form of distribution.

- Therefore any fork that implements, for example, a change to the programming language itself, but still uses the name "Rust", is distributing a modified version of the programming language in a manner that is not allowed.

I sincerely hope that this is not the interpretation taken by the Rust Foundation, but I cannot know for sure. It seems very open to selective enforcement.