You ought to know the difference. Hasn't the country been talking about it long enough.
And no, the slave system didn't take shape "by the free choice of those concerned in it."
But that doesn't change the basic point that the Deep South states were as dependent on New York (and the North and Britain) as anybody else anywhere was dependent on them.
Indeed, regions that concentrate on producing raw materials and don't develop industries and leave their financial affairs in the hands of others, are more dependent than those who diversify their economy.
You can see here on page 22 that in discussion of the advent of free trade, that he does not make the argument you evoke. here
He did go on to say on page 57 that "Slavery is a great institution."
He must have been refuting Kettell?