They can't relicense contributions, but they can add new GPL-licensed contributions. That makes the whole work a derivative work in the sense of the GPL, which means the requirements of the GPL apply to all of it. Effectively this is the same as if all the contributions were relicensed under the GPL. The only significant difference is that you can still cut out a part of the code that hasn't been touched by a GPL-licensed contribution and use it under the terms of the MIT license. None of this violates the MIT license of the original contributions, as it allows you to do basically anything, including sublicensing, with the code, provided that you retain the copyright notice.
Comments
They can't relicense contributions, but they can add new GPL-licensed contributions. That makes the whole work a derivative work in the sense of the GPL, which means the requirements of the GPL apply to all of it. Effectively this is the same as if all the contributions were relicensed under the GPL. The only significant difference is that you can still cut out a part of the code that hasn't been touched by a GPL-licensed contribution and use it under the terms of the MIT license. None of this violates the MIT license of the original contributions, as it allows you to do basically anything, including sublicensing, with the code, provided that you retain the copyright notice.