سَيَقُولُ ٱلَّذِينَ أَشْرَكُوا۟ لَوْ شَآءَ ٱللَّهُ مَآ أَشْرَكْنَا … قُلْ فَلِلَّهِ ٱلْحُجَّةُ ٱلْبَٰلِغَةُ · قُلْ هَلُمَّ شُهَدَآءَكُمُ ٱلَّذِينَ يَشْهَدُونَ أَنَّ ٱللَّهَ حَرَّمَ هَٰذَا
"Those who associate partners will say: 'Had God willed, we would not have associated partners'… Say: 'The conclusive argument belongs to God.' Say: 'Bring your witnesses who will testify that God forbade this.'"
The same logic was treated at Yāsīn 36/47 (law yashāʾu'llāhu aṭʿamah) and it was recorded there: the premise is true, the conclusion false — what is criticised is not the belief in destiny itself but its being used to escape responsibility. I rely on that treatment.
And the answer is worth recording: the verse does not enter into the argument — it says fa-li'llāhi'l-ḥujjatu'l-bāligha. And after that it passes to the second question: hātū shuhadāʾakum.
I record this as my own reading: instead of carrying on the argument about destiny, the sūra turns to a concrete demand: a witness as to who laid down the prohibition. And this is the knot of the whole last block of the sūra — indeed the next verse will give the real list.