Chris has never believed that Sonichu existed in this world, but for decades he's unironically declared that not only does Magi-chan exist irl, but that Chris can see him and talk to him, even touch him and everything else than implies. He has never implied this about Sonishu as far as I know, other than briefly saying he was possessed. So if there was an entity that could jump between hosts and affect other people, it most probably wouldn't be the entity that is supposed to exist only in another world, it would be the one that was supposed to exist in this world and act independently. The most consistent icon Chris has worn has been the medallion, so even if it isn't in the form of Sonichu, it could have attached itself to it.
But most importantly, at the time Chris was arrested, he had claimed he was possessed by Magi-chan, not Sonichu. When Null talked to Chris, he had demanded to be referred to as Magi-chan, and in the posts made after Chris was supposedly restored to his body, Magi-chan claimed to still exist in this world.
The origin of the demon theory and its evolution before Rekieta made it far more famous is from this anon and then Audentum, you can go through at least
Audentum's video to understand most of the theory.
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