To all intents and purposes it is waterproof - the liner is floating (not sewn into the outer cordura bag apart from at the top where the zip is) so water can't seep through seams or stitching.
Water would have to get through two layers of PU-backed fabric or through the water-resistant zip, which is folded under the flap in normal use and not exposed to the outside. If the bag was submerged water would probably get in but for on bike use it is about as good as it gets.
With the flap up I guess the zip is exposed but it is a small target and seals tight (if there is air inside the bag when the zip is done up it requires effort to force it out). As the bag is less comfortable when used in that way it's not something you'd do often, and mostly for things like a big shop at the supermarket.
I've certainly never had any water get in even in heavy rain. Obviously the bit between the pocket and bag where a lock would go is wide open to the elements though.