They address this and find the same gap in the normalized subset of rural vs metropolitan areas. The gap goes from 5 point to 66, which is slightly less.

democrat 143 counties, n=91 809 974 to 156 counties, n=133 796 619 And republican 292 counties, n=61 407 202 to 280 counties, n=46 244 883

So overall health is improving, but it is improving more when increasing sample size. It is normalized, but still different things are compared.

Thanks I didn't know all that about democrats and counties, seems more involved then I thought.