The leading Founders were learned and thoughtful men, and they lived at a time when the word liberty was on everyone's lips. They thought slavery would go away on its own. Once they Revolution was over and things calmed down, it was hard to overcome objections and work out the details. The book, though, shows the objections as well as the hopes for emancipation.
They were wrong. I think it's important to admit that succinctly. There's a huge gulf inbetween a miscalculation based on reason and hope on one side and a cold, calculated racist agenda on the other.
Admitting that they were wrong brings it full circle in a meaningful way. It's also important to bring up the early American abolitionists, crossing over or not with some of the Founders; because they too thought it would go away - not on its own, but that slavery would go away if the trade were abolished. You see this in the early pamphlets. You can see it in Benezet's influential "Account of Guinea", for example, all about the slave-trade. There were plenty of full-fledged abolitionists to be sure, but they knew they had to get focused if they wanted to achieve success. British abolitionists a few decades later faced the same problem. You can't get all of slavery, let's just get the trade.
They believed that a lake cannot stay wet if its feeding river is eliminated. They too were wrong, those original abolitionists. That's why there's a two to three decade lull and full abolitionism returns to the scene. They did successfully get rid of the river, and waited around to see the lake dry up.
To their horror, it didn't dry up. It's terrible, but that's how the events actually happened.
"Once they Revolution was over and things calmed down, it was hard to overcome objections and work out the details."
Agreed. Its easy to rally people (who are already fired up in the first place) against slavery when it's that evil empire's slavery, brought here by some foreign entity. Maintained by a tyrant's legislative veto pen.
The Founders were not evil socialists bent on re-structuring society at all turns, using government coercion and punishment against their enemies to achieve their purpose. The king vetoed anti-slavery, so that's the situation the Founders found themselves in and had to work within it.
Without throwing thousands into gulags for wrong-think.