Miss Marple, they can't on the one hand be forgeries and on the other hand be claimed as the private papers of the late Col. Killian. They are one or the other, but not both, unless it is alleged that Killian created the fraudulent documents. This statement sounds like the family is trying to cover all their bases because they don't know for certain that the documents are frauds. They may not have intended it that way, but it's how the statement reads.
The family didn't make the statement- it's just some freeper's "suggested statement." At least that's how I read it...