You are reading a tafsir for the group of verses 21:3 to 21:4

أَفَتَأْتُونَ السِّحْرَ‌ وَأَنتُمْ تُبْصِرُ‌ونَ (Would you then go along with sorcery while you have eyes to see? - 21:3) Those people used to discuss among themselves secretly that the claim of the Holy Prophet about his prophethood should not be accepted, because he was an ordinary human being like all others and not an angel. On the other hand not even the most rigid among the unbelievers could deny the supreme charm and eloquence of the Book of Allah nor its power to influence people when it was recited before them. Therefore, in order to turn people away from the Holy Book they started calling it magic and black art. They thought that they could keep people away from Islam by labeling the Qur'an as a book of magic thereby dissuading people from going to the Holy Prophet ﷺ and listening to Qur'an. Probably they used to discuss this subject secretly among themselves lest the Muslims come to know about their foolish views and unveil their fallacy.