And those who, when an oppressive wrong is done to them, take revenge.
means, they have the strength to take revenge on those who commit aggressive wrong and hostile acts against them. They are not incapable of doing so and they are not helpless; they are able to take revenge against those who transgress against them, even though when they have the power to take revenge, they prefer to forgive, as when Yusuf, peace be upon him, said to his brothers;
لَا تَثْرَيبَ عَلَيْكُمُ الْيَوْمَ يَغْفِرُ اللَّهُ لَكُمْ
No reproach on you this day; may Allah forgive you. (12;92)
even though he was in a position to take revenge on them for what they had done to him.
The Messenger of Allah forgave the eighty people who intended to do him harm during the year of Al-Hudaybiyah, camping by the mountain of At-Tan`im. When he overpowered them, he set them free, even though he was in a position to take revenge on them.
He also forgave Ghawrath bin Al-Harith who wanted to kill him and unsheathed his sword while he was sleeping. The Prophet woke up to find him pointing the sword at him. He reproached him angrily and the sword dropped. Then the Messenger of Allah picked up the sword and called his Companions. He told them what had happened, and he forgave the man.
There are many similar Hadiths and reports. And Allah knows best