That’s silly but after the warranty it can be safely done but not for amateur owners. if properly done theres no harm to the batteries. Disinformation is not becoming.
I carry my second battery on my rack. I do not yet have a battery cover for it. The battery locks in and works fine in the Aventure (version 1) without the battery cover. You will lose a lot of water resistance though without the cover. I do use velcro straps around the downtube whether I'm using the battery with the cover or the one without it.