"........they have discovered a new way to attack tumors that have learned how to evade existing drugs."
Does this refer to tumors already in a person or is it talking about the ones formed initially?
How would a tumor develop a resistence to a cancer treatment unless the cancer had been previously exposed to the drug. If that's the case, cancer would have to be a disease that's transmitted from person to person.
The immunity comes from evolution through natural selection. When someone with cancers is given chemotherapy, cancer cells die. But not always all of them. A few cancer cells may have a variation that lets them survive the chemo. They continue to reproduce and the cancer comes bsck. This time, though, the cancer is descended from the few cells that were immune to chemo the first time so repeat chemo has almost no success the second time around.