It wasn't quite enslavement. Earth was going to be a vassal planet to the Salaarnien Authority and would enjoy the benefits & protections of being a member world but have none of the rights of a founding or joined world. Also advies of any age and ability could be conscripted into military duty or galactic betterment services, both of which involve a great deal more than their names imply. Much of Earth's industries would be bent towards supplying other member worlds with various goods and most of Earth's normie population would be forced to help to produce those goods. But otherwise everyone could enjoy life as they had known it before being conquered. And could go anywhere they wanted among the Salaarnien worlds. 

It's definitely not a great deal. But enslavement is hyperbole and simplification. Sybil still gets her point across though.