What is meant by their "witnesses" who are mentioned in the verse?

Imām Ibn al-Jawzī (d. 597 AH / 1201 CE) summarized the scholars' opinions in his book "Zād al-Masīr":

1. They are their false gods. [Ibn ʿAbbās, al-Suddī, Muqātil, al-Farrāʾ] Ibn Qutaybah said: They were called witnesses because they witness them and are present with them. Others said: They are called witnesses because they worshipped them so that they would testify for them before Allāh.

1. They are their helpers. [ِAnother narration from Ibn ʿAbbās]
2. It means: "Bring forth people to bear witness that what you produce is like the Qurʾān". [Mujāhid]