As it's under MIT-license, they can't buy the software itself

This is a common misconception. Just because the author or original IP owner chose to release it under MIT doesn't mean they no longer own it -- and if the company wants to acquire it they can. They can never remove-from-the-internet the existing MIT-licensed versions (though they could take down their own copies), but being the owners they could certainly decide to release future versions under a different license.