Type: Null, and by extension, Silvally, weren't created with the intention of an exact replication of Arceus. Rather, Sinnoh folklore lead to the Aether Foundation creating a Pokémon with inherently similar abilities to Arceus.
BKP Development Record #5
Materials for development have been collected from the Canalave Library in the Sinnoh region.
Plans are being drawn up for the specifications of the Type change program, known as the RKS System.
Work is underway on designing the specifications of special memories for said system.
The Beast killer project was ran by the Aether Foundation to create a Pokémon to fight Ultra Beasts. Research for it was pulled directly from myths and legends in the Canalave City's library in the Sinnoh region, heavily implied to be about Arceus, and inspired by its ability to switch types. However, unlike Mewtwo being a direct clone of Mew or Regigigas shaping the legendary giants in his image, the Aether Foundation has research and folktales on a supposed mythological god fuel their ambition for creating a new, powerful species. Evidently, reckless scientists creating a new, powerful species in the image of a god, for research and/or power, is a trope in science fiction, so the connection is likely intentional. Though Arceus is in fact a Pokémon, the characters in-universe, especially outside of Sinnoh, wouldn't have direct confirmation of its existence.
Going beyond the lore, Type: Null/Silvally possess visual similarities to Arceus. Beyond being a white quadruped, the closest visual similarity I find is Type: Null's control helmet having a seal that resembles Arceus' cross-like wheel with four green jewels.
There's also precedent with in-game factors too. Both are Normal-type's that change type and appearance with a held item (Memories or Plates) in conjunction with an ability (Multitype or RKS system, which is phonetic for Arceus) and their signature move's type (Multi-Attack or Judgement). They both have the same base stats across all stats, though Silvally's is significantly lower.
Bulbapedia - Beast Killer Project
