Perhaps Sanders just thinks reparations are bad policy on the merits. There are many plausible reasons that a principled radical might come to that conclusion (though it isn’t entirely clear to me that Sanders is that radical even on matters of class).

Perhaps he is convinced that the highest incarnation of justice is a government that redistributes resources toward its citizens based wholly on their need, and doesn’t want to shift the Overton Window toward any model that is predicated on dessert beyond need, or that would redistribute wealth from poor to rich in some instances.

That seems consistent with principled socialism.

Perhaps when Sanders says that reparations would be divisive, he doesn’t mean that they would damage his campaign or the Democratic coalition by dividing its supporters––the plausible interpretation that Ta-Nehisi argued against in his critiques––but that it would divide Americans of different races against one another in a manner likely to cause more harm to vulnerable minority groups than good, or necessitate a divisive process of bureaucrats defining who qualifies as black. Maybe he was thinking that reparations poll dismally when their terms are undefined, and that hashing out specifics (I’m not sure if Ta-Nehisi wants Sanders to embrace the policies suggested by his 2014 article, or as popularly defined) would be divisive even among those on the left who favor reparations in the abstract.