That does not discredit what they said or thought on this issue. This is essentially an ad hominem.

It matters when you are pointing to the 'hominem' as political thought leaders. I think it's fair as a counter to an appeal to authority.

It would only be fair as a counter to their authority if the attack is relevant. I don't think it is relevant to drag slavery into a discussion about the nature of democracy and mob rule, or that the existence of widespread slavery on every continent at the time detracts from their insight. It is a purely tangentual argument to be had. For all your moralizing, I know for a fact that a vast majority of the whiners today would not have had the courage to fight slavery when it was happening literally everywhere. The ancient Greeks and the founders of the US have in common the fact that they laid the groundwork for the slavery-free condition we enjoy today, whether you like that or not. They were the geniuses of their time, and outstanding among all geniuses of all time.