Earlier quoted context omitted.
Nowhere in the article does it suggest that anyone lied about the books.
It doesn’t need to. Pretty clear from reading it that no one wanted to admit they’d been stolen on their watch, so they kept up some kind of pretence of it being lost within the library so they wouldn’t be public hounded for being responsible for its loss.
What do you want to happen when unique documents disappear? Call the cops? International press conference? I assure you that professional pride is much more important for these people than being 'hounded' by the public.
While there is a good chance that it was been stolen, it could still turn up . Things can go missing for, literally, centuries in collections that are big enough.