The shiny arceus event IS legitimate. It is actually one of 19 different arceus when you do the event. What?! Yes. The first 17 are holding a type plate with judgement, earth power, blast burn, hydro cannon. The bolded moves are event exclusive. The 18th is a normal type arceus with silk scarf and the same four moves. The last one is, as you saw, a shiny arceus. It is exactly the same as the normal typed arceus, just shiny. However, getting one of the 19 arceus is RANDOM, so good luck getting a shiny!
In conclusion, this arceus QR code is legitimate.
Source: serebii.net (scroll down some to find the info)