No, you're not misunderstanding. Unfortunately I'd say they are somewhat delusional...this just isn't the way people and relationships work long term. I'm fine with the no secrets part. But i's just not practical for all three of you to be together all the time. Things will be said to one person that are innocently forgotten and omitted when talked to the other. As I said, I've seen this before. It leads one of two things, from my observations : 1) accusations of lying because you can't remember absolutely everything you said to one person or the other or 2) the couple figures out that you can't keep this kind of thing up as time moves along and they give it up.
I wish you luck, DN. Relationships are never easy.