There were over 1000+ commits made in over a year -- all of those changes and improvements are GPL. (You can make GPL mods to MIT code and release those all as GPL.) If Wix had used the original ZSSRichTextEditor they would have been fine, but they didn't.
The key thing to understand is that being “compatible” with the GPL means that you can take code written under an MIT license , and put a GPL license on it if you want to. The MIT license allows for this, but, the reverse is not true. The GPL does not allow you to take GPL code and change the license to MIT, and it definitely doesn't allow you to take open GPL code, and make it proprietary.