Taxation and representation were directly linked. Because the South controlled the central government, it controlled the taxing power. When that went away, the possibility arose that slaves might be taxed directly, and since in the late ‘50s, an able bodied male slave was a very valuable piece of property, that could hit the slaveowner pretty hard, even paying for three in five would have been a real hit. The slave states seceded because they feared a central government they could no long control.
Certainly true.
So long as we remember that: first, it was all about slavery, and second, it was all about the Slave-Power's fears of what might happen at some point in the future, not some grievous "oppression" already happening.