I've always been unsure of how to feel about this statement. Yes, the abomination that was slavery did build the wealth of rich Southern landowners, but, that in it's own did not build America, in fact the opposite is true as the only people who benefited were white slave owners. The railroad that linked the Eastern USA to the west of the continent was built using Irish and Chinese labour and the coal was mined by Welsh coal miners and many Welsh came over and worked in the iron and steel industry.
The truth is while the immigrants who built the railroads and made the materials that allowed the railroad to be built were poorly paid, they were free and that was more than the slaves had, but, one group is not responsible for the economic powerhouse that America became, they all were.