Comment by greatgib

Even if the repository was closed to the world, it was open to some individuals. Some individuals got the work under the MIT license that allows them to do whatever they want, including opening it to the world.

What is not right is that once it was MIT with contribution from multiple authors, it is theoretically not possible (even for the original author) to change the license to something else than MIT without the agreement of the other authors.

Things would have been different if it was the private repo of a company only used by employees of this company, and having contributors bound by a special contract like a NDA.