Ericcc:
The type-changing effect is an effect of the move Judgment, not of the ability Multitype or of the species Arceus. That's why Judgment's description reads "If user is holding a Plate, this attack's type depends on the Plate held." and not "If user's current species is Arceus and is holding a Plate..." or "If user's ability is Multitype and is holding a Plate..." Thus, even a Smeargle can have Judgment's type change if it holds a Plate.
The type-changing effect is an effect of the move Judgment, not of the ability Multitype or of the species Arceus. That's why Judgment's description reads "If user is holding a Plate, this attack's type depends on the Plate held." and not "If user's current species is Arceus and is holding a Plate..." or "If user's ability is Multitype and is holding a Plate..." Thus, even a Smeargle can have Judgment's type change if it holds a Plate.