I heard another interpretation
...
The problem with slavery is that you can capture a highly skilled slave (though this alone assumes that you are invading a highly developed country) and he will likely work for you but you can't raise skilled slaves...
Yes that interpretation is wrong grasping onto tangential details while ignoring actual cause.
It is certainly possible to build a society that raises skilled slaves. This was the structure of ancient Egyptian society. What you cannot do is expected such a society to be competitive with one that frees its skilled labor.
I covered the relationship between freedom and morality in the following posts.
Religion and ProgressFreedom and GodFreedom is Approached not Achieved