I have no idea why anyone would claim the devil was "destroyed" before humans were created. Talk about ignoring scripture from Genesis to Revelation.Before the foundation of the κόσμου [kosmos, world, system as translated by scripture4all implying order, configuration as translated by Westcott and Hort implying order in order for the Son of God to be manifested as the Son of Man to destroy the devil and his works would it require the creation of man in the image of his creator yet subject to, the death, brought fourth by the sin?
Did the devil and his works need to be destroyed before man was created?
As for the "down casting" of the "Kosmos" (humanity) we can read about it in Genesis. Adam and Eve were cast out of the garden due to the consequence of sin.