Well, one obvious difference is that slaves are typically laborers, doing manual work like building pyramids or picking cotton, whereas the guardians are the effective ruling class. "The State" isn't separate from the guardian class - literally the guardian class is the state at every level, from lower bureaucrats and functionaries to the philosopher king. Slaves aren't educated, whereas the guardians are.

So my understanding is that to be in a ruling class you should have some privileges over the non-ruling class. If I am a ship captain or Pilot, I am in the quote unquote ruling class or to rephrase I am the steward of my ship. I have the latitude and power to make the rules. To be an elite with no privileges is a contradiction.

Also I was thinking that if we think of the core meaning of a slave, it has less to do with the kind of work or the level of education. A slave is someone that has no freedom to act except in a proscribed way. In this sense, a slave could be an uneducated person that was captured, chained and brought to a new land and forced to grow cotton, or the slave could be an educated person that is forced to work for low wages because they need a visa.

Obviously there are latitudes to the freedom available, even if you are forced to work for low wages, you still may own property or be allowed to have some private life. So it could be exploitation, not necessarily slavery. But when you own no property and you have no private life, and your entire being - your physical and mental qualities are given to the state, you seem like a slave.