Even leaving aside the absurdity of the suggestion that military victory could settle a legal issue (let alone a moral one) – isn’t it another establishment mantra that the Civil War was solely about slavery?

They seem to be trying to have it both ways. If the Civil War was solely about slavery, then the most that it could have settled is the illegitimacy of secession-to-protect-slavery, not the illegitimacy of secession per se. After all, present-day secession advocates are not exactly trying to protect slavery (unless Kirkpatrick Sale has a secret agenda we don’t know about).