This is some very bizarre reasoning. They still have their code. They are still free to do whatever they want with it. If they want my contributions in their product, they made a serious error in their license choice. Maybe they should have used the GPL.
But do you know what? They can use my code. They are completely free to use my code. As long as they only use it in GPL software.
"I do. Under the MIT license."
By saying this, you are saying to me that I am free to re-release your code under the GPL and add my own contributions. If you are not happy with this arrangement, you have made an error in your license choice.
I have not made any errors in my license choice. It behaves exactly in the manner I want it to.