What consequences would this theory of depression have for the goal of controlling akrasia?
The first question you ask should be something more like "Is this theory true?" "What do scholars say?" "Does this make any prediction that could be matched to historical (or even modern) accounts of slavery?"
Notice the "would". That makes it conditional on whether it's correct. S/he's not jumping to conclusions, at least not in that sentence. It's useful to ask that question (the one you're taking exception to), because it provides something to test. If there's a consequence from this theory of depression for the goal of controlling akrasia (e.g., the one he suggested), then one could test it out and see whether it works.
You could say that it wouldn't matter, and maybe it was just dumb luck that the theory generated a correct hypothesis, and it's still not clear whether the original theory is correct, but then the response is simply that one must run more tests.
Individuals who have to conform to an external authority too powerful to resist often get depressed: which among other effects, includes lower productivity and higher risk for suicide. This suggests one way how a tendency for depression, and resulting behaviors of akrasia and suicide, enhances survivability. After all, humans have always had to live with the threat of being conquered and subjugated by other tribes. A conqueror has a choice to kill a prisoner or to use them for labor. A prisoner who becomes depressed and thus poses a lower threat to the conqueror is more likely to be spared. However, survival as a slave is difficult if the master imposes too many hardships on the slave. Therefore, it makes game theoretic sense for a defense mechanism to exist which makes it undesirable for a master to make life too difficult for the slave, in the form of the effects of depression. The lowered productivity resulting from depression means that a master gets diminishing or negative returns from working his slaves harder. At an extreme, the risk of suicide means that a master who pushes his slaves too far risks losing them.
It would seem that such behaviors are less adaptive in civilized countries, where the risk of being enslaved is much lower. However, depression may still be of some benefit due to the fact that the master-slave relationship continues to exist, in diluted form, in hierarchical institutions.
What consequences would this theory of depression have for the goal of controlling akrasia? Firstly, it suggests that the individual experiencing an impaired ability to realize certain goals first ask themselves, "Are these goals really mine, or were they imposed by external authorities?" If so, perhaps being able to convince yourself that your goals are really what *you* want would help motivate you towards achieving them.