Robin Hanson has amended/refined his thoughts about baseball and slavery:

Added 10a: It is possible to be sold into slavery, or to sell oneself into slavery, so up front compensation is consistent with slavery. The key is that while you are a slave you have little control over what you do. The “degree” of slavery is set by the size of the penalty if you don’t follow orders. A death penalty makes for a strong slave, while merely being fired from your current job with many similar jobs available makes for a rather weak “slave.” In baseball, the penalty is pretty big — never again working in your chosen profession and life-calling, and having almost no prospect for anything remotely as fun or profitable. For an analogy, imagine that if you don’t do what your boss says, you must to move permanently to a poor country where you don’t know anyone and have no unusually valuable skills. That is a strong enough commitment that I’d be tempted to call it “slavery.” Even though you still have a choice.

This point is mostly correct, almost enough to be banal. Essentially it seems to add up to "individuals in the labor market can face various levels of coercion" and "at the margins concepts like ‘slavery’ can get fuzzy." This is correct, and indeed regardless of wealth or status individuals in the labor market can still face coercion, which is I think what Hanson was trying to get at in the first place. And indeed, there is a long history of unfair and coercive labor practices in Major League Baseball. But I don’t think any of this necessarily adds up to slavery – it’s just a long history of unfair and coercive labor practices! If the bank forecloses unfairly on the only black homeowner on a block full of white homeowners, that’s unfair and cruel but it’s not ethnic cleansing. This is ethnic cleansing.

I understand and empathize with Hanson’s desire to challenge the boundaries and limitations of our thinking. But I also think it’s valuable to draw lines at some point even if it’s inevtiably arbitrary. There is a certain accumulation of unfair and coercive labor practices that amount to slavery. Exactly where to draw that line can be challenging, but I don’t think Charles Comiskey was a slave-driver. He was just a jerk.