A lot of the time, things are thought to be true by a lot of people while being completely false. I feel like this is likely an instance of that, especially with the demonic lore in the series and the callback to The 9th Circle. Did he say "All Hallows' Eve is non-canon" or did he say "Art didn't know he would come back to life because he was a normal human in the first film"? Sources and context is needed, I think. People claim a lot of things are non-canon without evidence.
Even if you interpret Art as being fully human and normal in the first film and supernatural in All Hallows' Eve, that doesn't make All Hallows' Eve non-canon. That either means the present day of All Hallows' Eve is in the future or Art's powers are not always present. I mean, he loses a lot of his supernatural power at the end of Terrifier 3. It's an easy thing to work around and there's no contradictions.