They did not provide at least 50% of the story or characterization, nor more than 50%.
Tron: Legacy is 125 minutes. That means the screenplay would be about 125 pages, because one page = one minute of screentime. So in order to get credit, they would have had to writtenat least or around 62 pages. If each of them had written "49.5% each," their names would be credited in the film...which they are not. I get what you're saying. "Maybe they wrote 57 pages." They probably didn't. If they had written that much, there would be credit.
There are a number of minor things they could have done. Fixed up some clunky dialogue, maybe made some changes to the first act to better introduce the characters, did some Act II work (it's generally considered the hardest part of a screenplay), altered or introduced the midpoint, maybe added to, altered, or fixed up some conflicts or resolutions, maybe changed a couple of plot points, made a more satisfying climax or set up a sense of future knowing Disney would be interested in a sequel...there are a ton of tiny yet important things. But they most likely didn't do anything major, considering they aren't credited.
I didn't mean to start a "thing." I was just trying to bring some fairness into the equation.