
For Aristotle, household management (the Greek term for which is the root of the word "economy") ranks higher than craftsmanship or production outside the household, since the latter exists to support the former, the former being essentially the "art of living". It's possible that since he defines property as anything that belongs to the private art of living, he is merely categorizing household employees as property by way of an awkward and trivial formality. But his elaboration on this problem is anything but trivial, as he is obviously very concerned about the question of how it is that some are subordinated to such a role and not others.
There is no denying, and Aristotle illustrates, the necessity of some kind of division of labor in which those with directive skills lead those with less directive (e.g. purely physical) skills, but even this division is problematic for him.
Proportionality is a recurring theme in Aristotelian philosophy as it is in Greek mathematics, and in this case what he sees is a problem of unknown proportions. Differences in physical ability are easier to see than differences in directive or intellectual ability, but even if we assume that some people are much better at direction than others, there is still a nagging problem. I may be as strong as an ox and the dumbest person alive, but my capacity for will and self-direction is still far superior to my merely animal capacities, and this is not the case in even the smartest ox. In other words, my innate capacity for self-direction, which ultimately is hard to detect, still potentially counts for a lot, even if it is less than that of another:

If this problem was so apparent to Aristotle, why has the question of human dignity (assuming that dignity is inseparable from autonomy) had to be remanufactured so many times since?
Furthermore, what happens to the question when it's recast in the language of ostensibly more "scientific" modern social science, which requires us to dismiss all teleologies as relative or misleading and all questions of value as irrelevant? They may well be irrelevant, but it is insufficient and unscientific to accept this second-hand. We must reexamine the original proposition and try to find out where and why it has failed, if in fact it has failed.