(Then, indeed, upon Us is their reckoning), meaning: their recompense, according to the tafsir of al-Suddi.
﴿ثمَّ إِن علينا حسابهم﴾ يَعْنِي: جَزَاءَهُمْ فِي تَفْسِيرِ السُّدِّيِّ.