﴿ وَهُمْ عَلَىٰ مَا يَفْعَلُونَ بِٱلْمُؤْمِنِينَ شُهُودٌۭ ﴾

“fully conscious of what they are doing to the believers,”

(And were themselves the witnesses) they used to testify that the believers were folk of error (of what they did to the believers) and it is said this means: they were present when they did this to the believers.