Even if you don't choose to believe what they did, you'll agree that Von Daniken is unlikely to die for his claims and doesn't claim (as far as I know) to have witnessed them. There is a difference.
Whether Von Daniken is willing to die or not for his theories is irrelevant IMO as to whether they have any basis in fact or not. IMO Van Daniken's 'theories,' have more credability than the one's recounted in the bible - especially the miracles.
No third party objective person/government
recorded any historical documents that 3rd hand verifies any of the stories in the bible. Even Jesus real existence itself isn't provable other than some vague references by the Romans.
At least in Van Daniken's case - he can point to a real stone, sitting on a real mountain - carbon dated 5,000 years old and ask the question, how the fuck did they do that with primitive tools.