I don’t believe that is correct. The north was wealthier and far more technologically advanced than the south was at the time of the civil war. The south had not yet industrialized and was dependant upon the north for many things, which is why the blockades were able to strangle them.
The northern states were far more industrially developed but not wealthier. There were more millionaires in the south but their wealth was tied up in slaves and land. Slaves were far more expensive than we tend to think today.
This is from David Blight’s War and Reconstruction:
“by 1860, there were more millionaires (slaveholders all) living in the lower Mississippi Valley than anywhere else in the United States. In the same year, the nearly 4 million American slaves were worth some $3.5 billion, making them the largest single financial asset in the entire U.S. economy, worth more than all manufacturing and railroads combined. So, of course, the war was rooted in these two expanding and competing economies—but competing over what? What eventually tore asunder America’s political culture was slavery’s expansion into the Western territories.”