I just finished my first 40k book, Horus Rising. The false Imperium, complete with 9 planets, yellow star, and Terra intrigued me. I was hoping that eventually some explanation would be provided: perhaps they duped the home system and forgot? perhaps they got lost in The Warp and truly believed they had returned home but everyone was gone? You get the idea.
But, at least within the pages of Horus Rising, there was no explanation ever provided.
Has the origin of "6319" ever been revisited or explained elsewhere in the 40k lore?