مِنْ مَعْشَرٍ سَنَّتْ لَهُمْ آبَاؤُهُمْ وَلِكُلِّ قَوْمٍ سُنَّةٌ وَإِمَامُهَا
They belong to a people whose ancestors established a way of conduct; every community has its customary path and an exemplar of it.
Sannat, from the root s-n-n, here means to establish a customary way. Imām describes an exemplar or leader of that way.
This is a description of inherited tribal practice. The word sunnah is used in its general, customary sense, not automatically in the later technical sense of the Prophetic Sunnah.