Just because I acknowledge the relationship between capitalism and slavery does not mean I think capitalism should be abolished. Slavery (or its near-cousin, serfdom) was the mainstay of the economy throughout the history of civilization before the advent of capitalism. I do think we need to rethink the economic system in which capitalism (which is a form of production, not an economic system) exists.
You definitely make one undeniable point. Any sweeping statement has to be overstatement. I do think, though, that the "contest of power" (as Michel Foucault would say) between 'money interests' and the 'planter class' was definitive of the early history of this nation. It went back to Hamilton vs. Jefferson, large-scale development helped along by government vs. small-scale development that limited the need for governmental involvement (Jefferson's hypocrisy aside--though he did think slavery should and would eventually be abolished).
'Free markets' and capitalism are not synonymous, either. I define the latter as 'mass production of goods and services for sales in geographically extended markets'. It has been pointed out to me that plantations would be included in that definition.
My point is that capitalism, as the name says, is defined by the large scale of capital involved. Small-scale production for local consumption can be carried out in free markets.