The United States were hardly the only place where Slavery was accepted at the time, without the moral qualms that the writer suggests. Not only in Brazil, until the 1880s, but in what was to become French West Africa, the labor system was completely accepted. (For example, in 1880 Senegal, the jewel of French West Africa, the population consisted of two classes, slaves and slave owners. When both Clinton & Bush II went over to Senegal and denounced "slavery," they were almost certainly addressing the great grandchildren of the slave owning class, who hoping for American dollars, politely listened, probably without informing their guest speakers of the reality.
As for General Lee's oath: It was to the Constitution, not to someone else's interpretation of where that loyalty actually lay, under the circumstances of Virginia's secession. The writer, here, arrogantly is assuming an interpretation of duty, that is not as obvious as he imagines.
General Douglas MacArthur--certainly an authority of Military Duty--treated both the Blue & Grey with equal respect in his classic Duty, Honor, Country.
Virginia's ratification of the Constitution was never understood as a permanent pledge to support all future office holders of the Federal Government. Such a theory would fly in the very face of Jefferson's definition of legitimate Government in the Declaration.
The writer is seeking to rationalize a result that he apparently desires. The piece does him no credit, whatsoever.
While I am a comparative newcomer to these Civil War threads I have noticed that there are far more attempts to disparage the Union soldiers than I've ever seen directed towards Southern ones. To hear some people all Union soldiers were Irish immigrants forced into the ranks right off the boat, rapists, conscripts, or just plain cowards. Grant was nothing but a drunk. Sherman was a pyromaniac. And it was just blind luck that the North won any battle at all much less the war.