Can you produce Scripture which says that Christ was slain before the foundation of the world? Obviously, it was planned before the foundation of the world but I don't know of any Scripture which says He was actually slain before the foundation of the world.
I think the verse he has in mind is:
Rev 13:8 And all that dwell upon the earth shall worship him, whose names are not written in the book of life of the
Lamb slain from the foundation of the world.
I don't think anyone is saying that Christ WAS actually
dead, or "considered dead" (crassly), or what-have-you, before time began.
Nor do I think anyone is trying to diminish the physical act of sacrifice, or laying down life-for-life (or -lives) when they point out that the Incarnation was an act that prepared the Son of God for death. That little baby in a manger was there for one reason only--to DIE.
The Passover lamb was
chosen on the 10th of the month. Then it was killed on the 14th, at twilight. Why was it set apart 4 days earlier? Because preparation was part of the event. Perhaps we accept that Jesus didn't receive the "weight" of our guilt prior to his actual crucifixion, although there certainly seems to be a "process" to the pouring out of the wrath of God that includes Gethsemane and his trials.
Why don't we just acknowledge that this is all a pretty overwhelming scenario that defies neat categorizations?