I'm hearing from what you've said here that he claims the undead were natural. They're not. They're very rarely even the same spirit they were when alive. Ellethia broke through the mirrors into a realm of the dead — we can speak with Zenobius, you and I, if you like.

Matthias lost his daughter to natural causes while they conducted their research. The doll was originally a gift from his fellows, who was... possessed after, much as I could tell.

He's orchestrated lies for his own ends before. Including the undead lords like those we've faced already, and one killed.

I can send spirits beyond the living world. I have, more than once. There's nothing natural in what these dead are.