The picture was not saying underprivileged people can't be racist or sexist, ever, only that they cannot be equally so because of societal inequalities that stretch beyond the prejudices of individuals.
To simplify, because that's apparently something I have to do: Person A and person B are equally able to punch each other in the face for petty reasons; however, person A and person B will not be treated equally by the system for it. If you still don't understand, then I'll be done. I'll have tried, and I'll leave it behind.