It's a fitting description, but since Uber workers are paid I suppose a better analogy would be "master/servant".
Not being able to quit as you were owned by your master made one a slave. Slaves were often paid to increase motivation.
I think that is the point that is trying to be made. If uber "captures" this industry they will become the "masters" it will be difficult to find work outside their network, giving you no choice but to stay.
Peasants?
Serfs is the most accurate
"Well, I do whatever he tells me to do."
"Where I come from, we call that slavery." - Wonder Woman.