Why weren`t Arceus affected by Trick?

Question

Today I had a battle with somebody that confused me greatly.
I used Arceus- Fire (holding Flame Plate) and my opponent used Weavile. Weavile used Fling (Iron Ball) and Arceus used Judgment, taking Weavile out.
My opponent sent in Rotom-W. He then used Trick (learnt later he had Choice Scarf) but it didn’t work. He managed to use it twice even, but Arceus still had its Plate.
Didn’t Trick work because of Multitype, or maybe the Fling?

Answer 1

Because of Multitype trick wont work. If it did arcues could change type mid-battle which would be too overpowered, being able to change its type to take on teams better.

>Multitype changes the type of the Pokémon with this Ability depending on the Plate attached. The item-removing effects of Thief, Covet, Trick, Switcheroo, and Knock Off will not work on a Pokémon with Multitype.-Source

Comments

Thank you, suspected that but it didn't say anything about it on the Multitype page.
http://pokemondb.net/ability/multitype

Answer 2

Trick doesn't work on a Pokemon whose Ability is Multitype.

Source: Experience.