Buy that?
At the time, I believe the most important single crop in the south was indigo -- concentrated in South Carolina and grown in the hot, humid coastal plain.
Very labor intensive -- moreso even than cotton.
Frankly, I don't know why we're still quibbling on this topic. On the one hand, it seems obvious that "totally" was an overstatement and thus inaccurate. And, by the same token, it seems obvious that -- even in the 1790's -- the economy of the south (defined as Virginia thru Georgia) was largely (mainly, importantly, strongly) reliant on slave labor.
Indeed, the best proof of that claim may be that the southern states were prepared to depart the Confederation over it. And they were doubtless the best judges of just how reliant their economy was on that "peculiar institution" at the time.
The most important (and first) commercial crop in the south up until the revolution was tobacco, concentrated in Virginia, Maryland and NC. The link in #67 addresses that.