In explaining its authority, the textbook locates the consensus of this community after the Prophet’s death ﷺ. During his lifetime, matters could be referred to his authoritative, revelation-guided instruction.
«فإجماع هذه الأمة بعد ما توفي رسول الله ﷺ في فروع الدين حجة قطعية موجبة للعمل»
Thus, after the Prophet’s death ﷺ, the community’s consensus in branches of religion is a definitive proof requiring action.
«فروع الدين» here concerns practical rulings of religious law. The author is not turning ordinary administrative agreement into binding juristic consensus.