That was how I understood it. I'm not sure that this is even relevant to my question though, as enslavement, capturing, and trading are all different functions. What exactly is the requirement to be sacrificed? Must the unit be the result of enslavement, or will any "foreign" unit do?
How would you get a foreign unit to a place of sacrifice if it isn't enslaved? Check the Units tab in the editor -- it let's you decide what the resulting unit from an enslavement will be.
Best,
Oz