By comparison in the War of Independance slavery was a very minor issue. Britain may have had it's abolition society by 1774 but it was hardly government policy by 1776. Had the revolution been delayed until 1807 then slavery would have been a major issue & the balance of right would have shifted to the British side. Equally had it been delayed that long it is unlikely that the northeren colonies would, by then, have supported an independance joined to slaveholding states.
Which is why the premise for the piece being analyzed here is bogus. If secession was for the purpose of keeping southern slaves in bondage then perhaps there is a slim rationale for the argument. Otherwise it's simply historical revisionism with a new twist. It's meant to justify actions which were not otherwise justifiable.
Pennsylvania also had abolution societies in 1774 and in 1780, once free from British domination, they ended slavery in that state. One must wonder, if the United States had not won its independence, would the Whigs have gained control over Parliment and brought on the reforms that ended slavery?