Horne is a clumsy propagandist and you've been thoroughly propagandized.
In this kind of history, all nuance (like Adams' views on slavery) is lost and all historical events are made to fit a predetermined narrative. In addition to being wrong, it's incredibly boring stuff.
Just because you view all history as the result of a "secret cabal of evildoers" (in this case, plotting to preserve slavery) doesn't mean I have to.
As you let slip here, Horne's writings aren't about history: they're a form public of agitation so that "the USA will do less racist shit". Which would be fine if he was a social critic or a politician. But he's been passing himself off as a historian.
Words have meanings. "Propaganda" isn't just funny stories; it is done for particular interests. You claim he's a propagandist, so you should identify which interests those are. Failing that, you could retract your vicious slur...
In this kind of history, all nuance (like Adams' views on slavery) is lost and all historical events are made to fit a predetermined narrative. In addition to being wrong, it's incredibly boring stuff.