In many places in the Quran, Allah (SWT) says:
مَا لَمْ يُنَزِّلْ بِهِۦ سُلْطَـٰنًۭا
For example in 7:33, 22:71, and 3:151.
Let's analyze the first verse:
قُلْ إِنَّمَا حَرَّمَ رَبِّىَ ٱلْفَوَٰحِشَ مَا ظَهَرَ مِنْهَا وَمَا بَطَنَ وَٱلْإِثْمَ وَٱلْبَغْىَ بِغَيْرِ ٱلْحَقِّ وَأَن تُشْرِكُوا۟ بِٱللَّهِ مَا لَمْ يُنَزِّلْ بِهِۦ سُلْطَـٰنًۭا وَأَن تَقُولُوا۟ عَلَى ٱللَّهِ مَا لَا تَعْلَمُونَ ٣٣
Let's assume that it only says وَأَن تُشْرِكُوا۟ بِٱللَّهِ وَأَن تَقُولُوا۟ عَلَى ٱللَّهِ مَا لَا تَعْلَمُونَ
The meaning is still complete. Do not commit shirk.
But when Allah (SWT) adds مَا لَمْ يُنَزِّلْ بِهِۦ سُلْطَـٰنًۭا
this creates a possiblity. It semantically means that Allah can permit shirk if he wants to, and for anything that he wants to.
An equivalent English example can be:
Do not eat chocolate
It's 100% clear and needs no further clarification. Yet:
Do not each chocolate, for which I have not given you prescriptions
This opens the door to the possibility that there can be prescribed chocolate.
How do we understand it linguistically? What is the tafseer of adding مَا لَمْ يُنَزِّلْ بِهِۦ سُلْطَـٰنًۭا
to prohibitions regarding shirk?