According to Mandela, both the oppressor and the oppressed require liberation.
Both the oppressor and the oppressed are robbed of their humanity. According to Mandela, if one person takes away another person's freedom, they themselves become a prisoner of prejudice. They become shackled and restricted by their own biases and actions.
While the oppressed are dehumanised because that is how the oppressors treat them, the oppressors too are dehumanised as a consequence of their own actions.
The oppressed may be seen as actual prisoners, but the oppressors too are prisoners of their own hate.