This method is literally killing the patient’s entire immune system by irradiation. They then replace it with a donor immune system that might in turn try to kill them.
This isn’t a good option for the vast majority of patients to wipe out a virus that can be well managed with drugs.
The reason that it was done here is that the treatment is intended to cure them of cancer, and as a side effect eliminates their HIV infection.
Whaaat…?