Determining mental illness from just being dumb or biased can be difficult and probably just lies in a matter of degree. Lots of people are wrong about things, and the most likely explanations are they are just un-knowledgeable about those things, or they are dumb. Lot of people have incorrect beliefs even when they have seen the overwhelming evidence against their beliefs, and the most likely explanations are that they are just very biased or very dumb (although bias is a form of dumb). But when you have lots and lots and lots of those wrong beliefs, in the face of overwhelming evidence which you have actually seen but still dismiss, and you largely can't discern the reality of the world around you, then at that point it is more likely in the realm of mental illness.
Conspiracy theory type stuff is usually believed by people that are very paranoid and very sensitive to not feeling like they have enough control over their own lives and destiny and it leads to an extreme bias which allows facts and evidence to be ignored. But throw in mental illness and you get the people who start believing all of the crazy stuff instead of just a little of it, and they start to take the opposite belief to that which is mainstream simply because it is the opposite of the mainstream and not because it makes any sense. Bottom line is when you are largely out of touch with reality as opposed to having a crazy belief or two, mental illness is likely involved. Also in the video Max says he doesn't believe atomic bombs have ever existed and they are also an elaborate hoax, and I have heard lots of his other beliefs along those same lines both in person and online. He is most definitely in the camp of being not just a little, and not just largely, but completely out of touch with the reality of the world around him.
