This is presented in the middle of the story to affirm the story itself. It is as if Allah, the Exalted, is saying to Muhammad , "Or do these obstinate disbelievers say that he fabricated this and invented it himself''

﴿قُلْ إِنِ افْتَرَيْتُهُ فَعَلَىَّ إِجْرَامِى﴾

(Say: "If I have fabricated it, upon me be my crimes...'') This means: such sin would be mine alone.

﴿وَأَنَاْ بَرِىءٌ مِّمَّا تُجْرَمُونَ﴾

(but I am innocent of (all) those crimes which you commit.) This story is not invented, or fabricated falsely. Because he (the Prophet) knows better the punishment of Allah for one who lies on Allah.

﴿وَأُوحِىَ إِلَى نُوحٍ أَنَّهُ لَن يُؤْمِنَ مِن قَوْمِكَ إِلاَّ مَن قَدْ ءَامَنَ فَلاَ تَبْتَئِسْ بِمَا كَانُواْ يَفْعَلُونَ - وَاصْنَعِ الْفُلْكَ بِأَعْيُنِنَا وَوَحْيِنَا وَلاَ تُخَـطِبْنِى فِى الَّذِينَ ظَلَمُواْ إِنَّهُمْ مُّغْرَقُونَ - وَيَصْنَعُ الْفُلْكَ وَكُلَّمَا مَرَّ عَلَيْهِ مَلأٌ مِّن قَوْمِهِ سَخِرُواْ مِنْهُ قَالَ إِن تَسْخَرُواْ مِنَّا فَإِنَّا نَسْخَرُ مِنكُمْ كَمَا تَسْخَرُونَ - فَسَوْفَ تَعْلَمُونَ مَن يَأْتِيهِ عَذَابٌ يُخْزِيهِ وَيَحِلُّ عَلَيْهِ عَذَابٌ مُّقِيمٌ ﴾

(36. And it was revealed to Nuh: "None of your people will believe except those who have believed already. So be not sad because of what they used to do.) (37. "And construct the ship under Our Eyes and with Our revelation, and address Me not on behalf of those who did wrong; they are surely to be drowned.'') (38. And as he was constructing the ship, whenever the chiefs of his people passed by him, they mocked at him. He said: "If you mock at us, so do we mock at you likewise for your mocking.'') (39. "And you will know who it is on whom will come a torment that will cover him with disgrace and on whom will fall a lasting torment.'')