That's an interesting point, and it seems correct. You need to consider the context and background to get an idea whether someone doesn't know better or actively tells the truth.

That's often hard for me with a lot of the "out there" medical advice, where it's not immediately obvious to me that they're just scams where the person goes home to laugh at the fools giving them money, but rather themselves believe in whatever theory they're advocating. I don't think it becomes an opinion (it's still a statement claiming to be factual), but it's not a lie, and certainly not blatant, though some of it is absurd (but again not to everyone, obviously).