How so?
No one disputes he was harmed.
That's precisely what they dispute. The court's argument is that Keyes had not even the remotest chance of winning the 2008 election, so he couldn't possibly have been harmed by having Obama in the election. Seems like airtight reasoning to me.
Where do you find fault with it?
Had Obama not been in the election, many things could have happened that never had the chance to happen. You can speculate, but you do not know with certainty, no one can, that the outcome would not have been a positive one for Keyes.