think you're reading a bit too much into the graph headlines. The plate tectonics creating a fertile region had a political consequence because it concentrated agricultural workers

It didn't concentrate them only in that area though. There are a lot of farms outside of that band. There were a lot of slaves outside of that band as well. Those regions had political consequences too and it wasn't because plate tectonics created the exact same conditions there. You're seeing labor struggle in a fertile area and drawing the conclusion that the fertile area was an ultimate cause to the struggle. Rather, there is struggle between slaves and slave-owners no matter the soil conditions. If this struggle happens anywhere, then the plate tectonics were coincidental, not causal.