If he is wrong in ways that could be honest mistakes then one should first try to point out the mistakes so he can retract his document until he can find stronger evidence or truly see the error of his ways. For that to happen people need to respond with rational arguments, not with outrage.
If he is wrong and does not respond to counter-arguments and -evidence, then some HR response would be appropriate. And note that I say "HR response", not dogpiling and not necessarily firing.
Argument-by-firing will only harden the ideological frontlines and further extremes.
So your point is that if a racist argument is partially right and good for an employer then it's all good??!
My point is that you should engage in rational discussion even if you think the other side holds an abhorrent, immoral, inhumane viewpoint. And there are many good reasons to do so. And just one of those many reasons is that the other side might be right about some things. Another is that engaging them in discussion is a much better approach of showing them where they are wrong, just punishing someone for holding unpopular views is unlikely to achieve that goal. Another one that I have not mentioned yet is simple reciprocity, you want others to extend that kind openness to discussion towards you when they think you're holding some view that is abhorrent according to their moral value system.
Well, if you feel confident that race-based slavery is in no way justifiable then it should be relatively easy to find arguments why it is not justifiable, no?
An obvious one would be pointing out largely overlapping intra- and inter-race variances in genetics.
If your race-based-slavery-advocate could actually show that specific races have attributes that make it the utilitarian choice to choose them then they would be making a valid point - on the assumption that it is necessary to have slaves in the first place - and you might want to shift to finding arguments against slavery in general instead of the slaver's specific choice of slaves.
Or you could argue that merit-based slavery is superior to using race as a proxy. E.g. genetic testing or performance evaluations of individuals.