Surely these facts do not discredit the American Revolution as a whole.
If the preservation of slavery had been the primary reason for the Revolution, it probably would have. But the colonists had other and more prominent reasons for revolting.
Dunmore's proclamation did apparently inspire Congress or Washington to open the Continental army to blacks, and the issue of slavery was less clear-cut and focused in the Revolution than it was in the Civil War. The possibility of emancipation after the war wasn't wholly excluded. It did happen in some states in the 1780s. It was far less likely that the triumphant Confederates would have taken that path so soon after the war.
BTW, thanks for bringing up this little-known part of our history. Judging by websites, it may be better-known in Canada than here.