From what I know of monocoque structures, Brent is correct when he says that the curved hull bottom is adding a lot of strength to the design. A sideways force on one of the keels is not just acting on a single
plane fulcrum as Dskira states but it comes close. The fulcrum is curved upwards at both ends making it harder to fold sideways. However, what is shown in the first picture of the cross section drawing does have a weakness if you discount the curved hull bottom. If that tank top were dead flat along its length with the keel
joint, it would act as a fulcrum and could potentially fold from a side force. Such being the case, I agree that the frames need to be tied in at the bottom but the way they are terminated at the tank top allows nothing to tie them together below the tank top so they must be connected across the top of the tank top. IF the frames went all the way to the center line, then web floors would greatly increase the resistance to a sideways bending moment but as designed that's not posssible. The frames terminate above the tank top.
Salty Monkey, the photos you posted of the interior
structure of that twin keel show a major difference between them and the drawing that Brent posted originally. They have the frames going all the way to the center line and joining. They resist any side forces by doing that because of the way they are all tied together, keel sides to frames to hull skin.