“CFA Jr. led an African-American cavalry regiment in the war and his experiences and reactions turned him against any notions of racial equality. “
I take it that you have never read the essay in question since it never once mentions African Americans, racial equality, or anything resembling the subject.
It’s entirely about the legality of secession and treason. In the original understanding of the American union as held by the South, versus the evolved understanding as popular in the North, and in regards to the United States own birth in the secession of 1776.
Adams's speech isn't a very convincing defense of secession or the Confederacy. It's more rhetorical than a close legal or historical analysis. Adams doesn't convince me that George Washington would have supported Southern secession. He also all but admits that the secession movement wasn't peaceful and was originally inspired by the desire to protect slavery and create a "great semi-tropical slave-labor republic."
But being from one of the First Families of Massachusetts, Adams had much sympathy and fellow feeling for the First Families of Virginia. Nice sentiments, maybe, but what he wrote and said doesn't amount to a convincing justification for the Confederacy.