I think you may have misread that part. He claims that the South wanted an immediate end to the slave trade, not slavery, while "the North" insisted it continue because it was the majority of their shipping trade.
I have no idea if any of this is true. The general POV is that the North wanted immediate end to the Trade, and the South did not, particularly SC.
And while the slave trade was important, I just don't think it had anything like the volume necessary to dominate the shipping trade, which employed a LOT of Americans. There just weren't that many slaves imported.
If I have time, maybe I'll go dig back thru Madison's records and see what really happened. Or at least what JM says happened. :)
South Carolina and Georgia opposed limits on the slave trade. Rutledge and Pinckney of SC even declared that South Carolina's support for the Constitution would hinge on this point.
Some Northerners like Roger Sherman and Oliver Ellsworth of Connecticut worked out the 1808 (originally 1800) compromise. Sherman wanted to keep SC and GA in the union. Ellsworth assumed slavery would eventually disappear (and wasn't apparently troubled by what happened in the meantime).
Now if one finds Virginia slaveowner George Mason passionately arguing for the abolition of the slave trade in the very near future and Northerners like Ellsworth and Sherman objecting one might interpret that as Southerners wanting the abolition of the trade and Northerners opposing it, but that would be a distortion and one would be wrong.