Along with "make impossible states impossible to represent". If you're designing your types to make a state unrepresentable, you'd better be absolutely sure the state really is impossible for the lifetime of the design.
I think what that quote is against is the common middle ground where states are expected to be 'impossible' and thus not handled and cause bugs when they are found to be not actually.
Either deciding that they are possible or must be impossible is usually better and which one to go with depends on the specifics