If dualism were correct then brain damage wouldn't effect cognitive ability. Simple as that.
This is a surprisingly common misconception. (Neuroscience student K.L. Dickson makes similar claims here.) It arises from a simple confusion of necessary and sufficient conditions. Dualists claim that the physical brain does not metaphysically suffice for mentality (cf. zombies). But Ookla's claim simply doesn't follow from this -- we may still think that the brain plays an essential, but non-exhaustive, role in constituting our minds.
A typical physicalist view is that the mind just is the brain. A dualist view is that the mind arises from the brain (given appropriate background conditions). Nobody holds that the mind is nothing at all to do with the brain.