Well, will try to analyze this.
We have two Arceus, shared by two users here. One on the PKX contribution thread and one on the .wc6 contribution thread (as a .pk6 file). The first one was not shiny and the second one is shiny. Here are the files: Non-Shiny, Shiny.
They have the following similarities:
They have the same SID / ID / OT / Attacks / Relearn moves, which seem are the real ones.
They have the following info in residence:
They both have the same memoy with non-OT (arrived via Link Trade / at Pokémon center / it felt good)
They have the same Nature (Rash)
They were both obtained in the same game (OR), same date (07/03/2015).
And finally, they have the same Hidden Power type.
And the differences:
They have a different IV distribution
The non-shiny one has the latest trainer value as Shaun while the shiny one Icarus
The non-shiny one was obtained in Chiba while the shiny one at Kyoto.
So, there's a possibility that the shiny one was based on the non-shiny one. However, we don't know if the original Arceus is legit or not.