What was the other 'Terra' system in Horus Rising?

Question

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?

Answer 1

No, there is no other information about the 6319 anywhere else, but many of the planets colonised before the Great Crusade have lost contact with Earth during the Age of Strife, when the birth of Slaanesh made any interstellar travel and communication almost impossible. Lots of words regressed, some even to the barbaric levels (in example even in the "present times" people living on Space Wolves home planet Fenris live with sword-level technology), so its not hard to imagine that there would be world that simply decided that they were the real Sol system.

It also seems obvious that this has been somehow orchestrated by the Ruinous Powers - in few years from that fight Hours will be storming the real Emperor's palace on Terra.