Who were those who said: "Then they returned to themselves and said, 'Indeed, you are the wrongdoers'"? What is the meaning of this verse, and are they gloating over themselves?

The oppressors (al-Dhalimun) are the disbelievers who championed idols. There is a difference of opinion regarding their oppression (zulm). It was said that they oppressed themselves by abandoning their idols, leaving them neglected and unguarded. It was also said that their oppression lay in their false worship of idols that neither hear, benefit, nor harm, and in their accusation against Ibrahim (peace be upon him). Their statement, "You have certainly known that these [idols] do not speak," is an admission from them that their idols do not speak, and this indicates their misguidance and obstinacy.