Rather, the commitment to not hiring slavery apologists isn't absolute. It should be treated like a real decision with costs and benefits, with the slavery apologetics considered a serious cost. If you could hire Bob or Steve, where Bob is politically "usual" and Steve is holocaust denialist, you should hire Steve only if he is a considerably better choice than Bob on "usual" grounds.
Edit: I would also add that hiring a slavery apologist when you already have one is a heavier cost still. These are not vacuum decisions. Similarly, if every one of your faculty has political views acceptable to either liberals or conservatives, you should reduce the "nasty cost" of hiring a fascist or a Stalinist.
you should reduce the "nasty cost" of hiring a fascist or a Stalinist.
The thing is current universities are perfectly willing to hire Stalinists.
Related: Heuristics for Evaluating the Soundness of the Academic Mainstream, Admitting to Bias, The Ideological Turing Test