MIT license is not parasitic like GPL. You can close an MIT licensed codebase, but you cannot retroactively change the license of the old code.

Stability's initial commit had an MIT license, so you can fork that commit and do whatever you want with it. It's MIT licensed.

Now, the tricky part here is that they committed a change to the license that changes it from MIT to proprietary, but they didn't change any code with it. That is definitely invalid, because they cannot license the exact same codebase with two different contradictory licenses. They can only license the changes made to the codebase after the license change. I wouldn't call it "illegal", but it wouldn't stand up in court if they tried to claim that the software is proprietary, because they already distributed it verbatim with an open license.