For the Stewards to be slaves they’d need to meet a few qualifications.
- Be a sentient race that has been forced, coerced, or obligated, into servitude without say, pay, or other factors that create an even field.
- Be against the set up that they find themselves in, and desiring to leave being forced to not do so.
- Be independent, with free-will.
The Stewards don’t appear to meet any of the requirements set forth above. They don’t want to not be doing their jobs; they exist for a purpose and are happy with it. They’re not independent of the system and are happy with it.
The Stewards understand and accept the position they hold in their society; they do not suffer it. The Stewards do not ask for more, but do not appear to be incapable of doing so, they’re simply not inclined to do so. They do not appear to have meaningful free will in that regard.
The difference between a human slave and a supernatural being who exists for a purpose and does that purpose are so vast that it’s hard to define. Calling them “slaves” is no more useful than calling humans slaves to their instincts for mating. You can warp it such that the definition fits but it’s so bent that it no longer matters as a descriptor.