Directors always have a group of people surrunding them and they are really doing all the actual gruntwork...
And Donner was there long enough to approve them calling it "The Donner Cut"
He was there long enough to sit by a microphone and record a commentary for the film.
Apparently this was at least long enough to sell fans on the idea that this was Donner's version of the film, and those fans start raisign holy hell on the internet that finally, the real sequel to Superman has been made, and how much better it is. They all drank the Kool-Aid, in my opinion. Even if it had been "finished" it's not a better movie. One only need to listen to Donner's commentary to realize that his attitude about the cut was all wrong.