I don’t get that. I mean, if we ignore the, y’know, enthused support for slavery as the best thing since sliced baby seal, he still supported nullification (which rather led to the fascinating question of what the federal government was for.) and the Calhoun Doctrine led pretty much directly to Dred Scott and the Civil War. His influence on the history of the United States can only be described as ‘ruinous,’ even if you ignore that his morality was clearly significantly under the ambient level. Even the people who were pro-slavery politically made some occasional nods (like Clay did, if memory serves) to the notion that slavery was fundamentally bad but, alas, necessary because Reasons. Not Calhoun. Oh no.
That said, I read the article and it seems that, despite what it was called it seemed to be selecting great senators where ‘great’ meant ‘of lasting impact’ much in the way you could describe Stalin as ‘great.’ Not good but someone who indubitably left his mark upon history. That’s more understandable. Calhoun was in politics for a very long time and he made good use of the time and did a lot: nearly all of it utterly awful but there you go.