لِأَنَّ ذَلِكَ إِنَّمَا هُوَ فِي بَلَاغَةِ الْكَلَامِ وَالْمُتَكَلِّمِ
For that consideration relates to the balagha of discourse and speaker.
Two questions need separating. First: To what subjects is the attribute applied? The text cites established usage as part of that answer. Second: What conditions define particular kinds of balagha? Here situational conformity becomes central.
An answer to the second question should not be treated, without qualification, as a complete proof of the first. Technical attribution and the detailed condition of a technical class are different issues.