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After quite a bit of research, I found they are legit. They are responsible for recent Gen VI events in Singapore, usually as English copies of Japanese events. They've done the shiny Rayquaza, shiny Mega Gengar, Garchomp, and a Hoopa before the west even got him.
For whatever reason, Serebii just doesn't have the Arceus in his database yet. As far as I understand, these serial codes work on both Japanese and American consoles. Though, their Facebook page is blocked to any region outside of Singapore. Perhaps a little effort with a VPN using a Singapore IP, you should be able to get a serial code.