Neither was it presented as the best example. One idea was taken from Athenian beliefs and op then says "It's through this lens that we should consider a wholistic approach ..."
Of course it's easier to throw out a random fact like 'oh slavery' so your argument is moot, but what are you really adding here?
It's not a random fact, it's the counterpoint to invalidate the OP's argument. It's easy to espouse individualism and not have to worry about anything when you have a whole class of people who you can order to do your bidding (to grow food, build buildings, even defend the city).
Of course it's easier to throw out a random fact like 'oh slavery' so your argument is moot, but what are you really adding here?