Some users have reported only receiving the 4 extra DLC cars in Rocket League: Collector's Edition for the Xbox One.
Not to fear, all of the 3 DLC pack items are there on your disc! The code included with your game will immediately unlock the 4 additional cars (Aftershock, Esper, Masamune, and Marauder), and as you play through the game, you will unlock everything else, including all of the DLC pack items from Chaos Run, Supersonic Fury, and Revenge of the Battle Cars.
Thank you for your support of Rocket League, and we'll see you out on the field!