They wouldn't. If you have it in a private repository and you are the only one with access to it, you'll likely not run into this issue.

From other comments, this developers "private" code was found in 30k+ public repositories with public attribution which is what created this issue.

Presumably your private code is not also present or leaked to any public repositories.

The, this brings up a very, very interesting question, people have stolen code or leaked it into public repositories, and Microsoft are building a product that references that code.