No. There is definitely not more rural votes to surge from than suburban/urban. To be clear, Cohn et al are not making that claim. They are saying that pro-Dem surges are tied to education & that the Midwest is lower edu than safe Blue states. And they're not wrong about that.
But those Midwestern states are not as low education as many other states either, including many that reliably break D and are even more white (NH, for ex). And what they are arguing is that in a high turnout environment, the low edu turnout will be heightened & non-college edu 
voters will make up the majority of the electorate. But keep in mind. Not all non-college educated voters are white and not all non-college educated white voters are breaking for the GOP-ESP ones that sat out 2016.
Also imp, even in a high turnout environment, we'll still be missing 25-35% of the electorate, and that missing component will be drawn disproportionately from non-college-educated voters. Further, these Midwestern states are fairly diverse, the least diverse is WI and I
expect WI to be the most competitive bc it is the least urban and most white. But as is the case everywhere, population density means there are much larger latent pools for Ds in most places. They are, by no means, anywhere even approaching maximization of the suburban surge.
