I meant that the origin story is a core element in their belief system, which is evident from every major christian religion has some teachings on this story.

If believers actually retreated to the position of invisible dragons, they would actually have to think about the arguments against the normal "proofs" that there is a god: "The bible, an infallible book without contradiction, says so". And, if most christians came to say that their story is absolutely non-empirically testable, they would have to disown other parts: the miracles of jesus and god, the flood, the parting of the red sea, and anything else that is testable.

That large sub-groups of Christians believe something empirically false does not disprove Christianity as a >whole, especially since there is widespread disagreement as to who is a "true" Christian.

I didn't say it would disprove christianity - I said it was a weaker form of the argument: there is an asymmetry between the beliefs of christians and evolutionists. But, most christians seem to believe that there is magic in this world (thanks to god). Sure, if they didn't believe it, they could still call themselves christians, but that type of christianity would probably not get many followers.