I think you're confusing this with the difference between having to make additions available as open source as well (like with the GPL) versus being able to kee

I think you're confusing this with the difference between having to make additions available as open source as well (like with the GPL) versus being able to keep additions closed source (like MIT or BSD). In this case, however, you're not allowed to distribute additions at all, unless you contribute them back to the original project. In other words, forking's not allowed. That's not commonly considered to be open source.