Perhaps you might think this is bullshit because *obviously* this world is real and not an illusion and there is *obviously* no evil demon to deceive us into thinking the Earth is spherical instead of flat.

And yes this is what philosophers do. Nobody here is arguing that such demon exists and is actually deceiving us, but since you've accepted you can't prove god doesn't exist (maybe mis-step for you since you're probably not the philosopher type), well, can you prove such demon doesn't exist? Seems to me the same thing.