The writer seems completely oblivious to the seismic shift between the antebellum 'these United States", of which Lee was a Virginian, first. The federation of those States paled in comparison to the loyalty owed to the Sovereign State from which Lee came, where family and property lay. To take up arms, no matter how prestigious that command, against his homeland would be an act of treason most foul, to take arms against his family, friends, and neighbors, an unthinkable act. Even then, Lee had sworn an Oath when he accepted his commission in the Federal Army, and doubtless that resignation was not a choice made lightly.
However, when the States were no longer united, when his homeland had voted to withdraw from the compact it felt was no longer in effect, to withdraw that consent of the governed that gave the Federal Government any just power over Virginia, as had other States, that Federation was dissolving. Virginia remained, his home.
It is amazing how many historians ignore the fundamental shift in mindset surrounding the Federal Government that occurred ante- and post-bellum, from a Federal Government to a National Government, a significant change which haunts this country to this day.
Slavery would have died an economic death with the influx of immigrants from Europe, and the simple economics of hiring someone rather than owning them.