True. But the point is, not counting them fully was a political calculation designed to marginalize the Southern states. Government policies developed in Congress tended to work to the advantage of the industrialized Northern states, encouraging manufacturing and more industrialization. The Southern states became simply a source of raw material to feed the machines. The North agreed to keep the agricultural economy of the Southern states that was designed around and dependent on slave labor as long as they needed to commodities it was producing.
So what prevented southern states from also industrializing?
At the Founding neither section was industrial.
Of course it was political. Were you aware that when it came to counting slaves for the purpose of apportionment for tax determination the south didn’t want to count them at all?