It's a technical issue. So read the article for that part. But the more substantive issue Cohn argues is that more white voters voted for Obama than we thought, not in the South but in the northern tier blue states. In other words, in just the states it looks like might decided the race.
When the Republicans did their post-2012 'autopsy', the point wasn't simply the growing population of non-white voters. The other premise was that Republicans - particularly Mitt Romney - had essentially maxed out the white vote. Obviously he hadn't literally maxed out the white vote. Lots of white people voted for Barack Obama. But lots of whites are confirmed liberals. So you'll never get all of them. The point was that Republicans wouldn't be able to run up higher margins from this segment of the population that was shrinking in relative terms.
The point of Cohn's article is that Obama got more white voters than we realized. And those were white voters, particularly in the northern tier blue states, that a different kind of Republican could drive up higher margins from. That basically seems like what Trump managed to do. There was a Latino voter surge. But Trump countered it by driving up the GOP's margin among white voters, especially non-college educated white voters and perhaps (this point simply isn't clear yet) bringing more of them to the polls.