This repo wasn’t actually relicensed. Here’s what happened:

• Kevin Bentley got permission from Parallax Software to release the source code for Descent 3, but Parallax Software didn’t say “you can release it under this particular license.” They just said “you can release it.”
• Kevin Bentley created a new GitHub repo. GitHub offered to automatically include a copy of the MIT License in the repo, and Kevin unintentionally agreed. This was an accident. (See 637df31 and this comment).
• Later, Kevin realized the mistake and removed the copy of the MIT License. (See 6a7a141).
• After talking with Parallax Software, Kevin got permission to release the code under the GPL (See 1f45163 and these two comments).

Why do you think that the MIT License would be a better choice than the GPL?