A reply invokes the relation of dependent and head (tābiʿ and matbūʿ). In several grammatical constructions a dependent element is not normally allowed to precede what it depends on. The analogy is then used to question indiscriminate fronting.
The counter-question is whether the dependency in each construction is genuinely alike. If the types of syntactic dependence differ, a rule from one structure cannot be transferred without argument.
The distinction is between ‘this element may occur earlier’ and ‘if earlier, it retains precisely the same grammatical role’.