The objection is that a relative clause is precisely a syntactic environment where clausal material is expected; the predicate position differs in its usual baseline form.
وَأُجِيبَ بِأَنَّ الصِّلَةَ مِنْ مَظَانِّ الْجُمْلَةِ بِخِلَافِ الْخَبَرِ
It was answered that the relative construction is among the places where a clause is expected, unlike the predicate.
The analytical lesson is that evidence should be transferred across syntactic environments cautiously. A superficially identical adverbial phrase may need an argument tailored to its grammatical position.