To keep playing the devil's advocate on this one, proving a negative is usually really hard. Especially when the subject clearly knows the content now. We're one of the best experts on Skaven in the Empire, if not arguably the best. It is really difficult to say 'I can prove that Weber did not know about the Skaven during the year of 2477'- especially when that's something like a decade ago now.
Off the top of my head I think Johan's the only one who could maybe sink that story, and Johan is a) a good friend of ours, and b) pretty savvy on this sort of thing for a gold wizard, so I don't think the risk is that large.