Is "indifferent" a postpositive adjective in "leaves no one indifferent"?

The adjective “indifferent” in that sentence just happens to come after “no one”, but is not attached to it. So it is not a postpositive adjective (which would be attached to the preceding material to form a phrase).

You can see this if you look at a related passive sentence: we would say “No one is left indifferent (by it)”, not *“No one indifferent is left (by it)”.