Translation (Maududi)
"Then they will be able to play no mischief but will say (falsely): 'By Allah, our Lord, we associated none (with You in Your divinity).'"
A staggering moment. Backed into the corner of judgment, with their false partners now visibly absent and useless, the polytheists resort to the only tool left — a lie. They swear by Allah, by their Lord, that they were never polytheists. Maududi notes the layered tragedy: they continue lying even at the Standing, even after death, even when the truth is manifest to all. The instinct to deny one's sin runs so deep that it persists into the afterlife. The word fitnatuhum ("their excuse, their tribulation, their mischief") is debated by mufassirūn — Maududi follows the reading that they realize at that moment that no other excuse will work, so they fall back on perjury.