Reading the article it states that the bell weather counties are whiter and less educated than the country as a whole and those are 2 of the main indicators if someone supports trump. Where as people of color and those with 4 year degrees generally support Biden. So they were bellweathers in the less partisan era of bush/Clinton/Reagan. But they actually started shifting in 2016 towards Trump even though he lost the popular vote but won presidency. And stayed with him during his loss.