Tafsir of Al-Mutaffifeen 83:33

This is a circumstantial clause referring to the pronoun in "they said" (qālū). That is, they said that while, in reality, they had not been sent by Allah the Exalted to the believers, nor were they appointed over them to record their states, oversee their deeds, or testify to their righteousness or error. This is a form of mockery and ridicule directed at them, signifying that the boldness they displayed in their speech is a task reserved for those sent by Allah the Exalted.

It is also permissible that this is part of the criminals' speech, and the original meaning would be, "And they were not sent as guardians over us." However, the phrasing "over them" (ʿalayhim) was used to convey the meaning, similar to saying, "Zayd said that he would certainly do such and such." Their intention behind this is to deny the believers' act of turning them away from polytheism and calling them toward faith.