Actually, he was born. All of the spartans, including john, were ready and had already done a few missions before the covenant first attacked, let only got to Reach. The battle on Reach led directly into the events of Halo 1. Chief and the Pillar of Autumn fled Reach with a random trajectory, and happened to find the first halo, so this game will still take place mostly in the same year as halo 1. All of the spartans, including Chief, were alive and fighting during the battle of reach, so i am 90% sure he will at least be in the game, maybe not playable, but at least in it.