كَذَّبَتْ ثَمُودُ بِطَغْوَاهَا إِذِ انْبَعَثَ أَشْقَاهَا
Thamūd denied through its rebellious arrogance when its most wretched member rose up. (91:11–12)
After stating a general principle concerning the soul, the surah presents a historical case. It is a practical illustration: abstract rebellion becomes concrete through a notorious act and its consequence.
Ashqāhā is a superlative construction highlighting a particularly wicked individual within the community. It does not assert that every member had an identical degree of wrongdoing.