It wasn't the subject at hand. It was an attempt to point out several factors largely ignored by those who wish to tear down America. Contemporary Amateur propagandists only choose to emphasize and exaggerate the worst aspects of slavery. This gives a slanted view of the real history. It also belays the tremendous pride solely American which should stem from being the first to put an end to the practice of slavery within a fairly short time after our founding. It contradicts the OP that America was built by slaves. Not so.
The system of slavery gave little to no incentives to slaves and they were much less productive than they would otherwise be like their free contemporaries. Much like people living under the yoke of socialism today, the system lacked any form of incentive for them to work hard for the most part. They lived without hope that their concerted efforts would result in any meaningful improvement in their standard of living. Hence no motivation, little effort. It wasn't until the abolition of the institution of slavery this segment of America began to contribute on an equal basis with White America.