To clarify, the modified HIV virus in this case was used as a vector to deliver an antibody specific to the cancer onto a T-cell. The T-cell then goes on to destroy the cancer. HIV is one of the most efficient vectors for genes.
EDIT: Also note that the HIV vector is used to modify the T-cell before it is inserted into the human. The HIV is never actually inserted into the human (in this case).
Such viral vectors are routinely used in the lab to knock in a variety of genes in the cell. Vector is really the packaging. What was important in the study was the actual antigen which has packaged in the vector.
http://en.wikipedia.org/wiki/Viral_vector
http://en.wikipedia.org/wiki/Lentivirus
http://en.wikipedia.org/wiki/Chimeric_antigen_receptor
http://www.nejm.org/doi/full/10.1056/NEJMoa1103849
EDIT: Also note that the HIV vector is used to modify the T-cell before it is inserted into the human. The HIV is never actually inserted into the human (in this case).