I know that's generally true, but apparently sometimes the odds of recurrence are so low they stop worrying about it. I have a relative who was diagnosed with stage 4 melanoma a decade ago. She got three doses of immunotherapy and her tumors immediately shrank by 2/3, and eventually disappeared entirely. For a while they kept scanning her twice a year, but several years ago they told her it was gone and stopped the scans.
That's great to hear but, your risk of cancer before diagnosis and after you're cured is, at best, the same.
Yes of course, even if all her cancer cells have been destroyed, she could still get an entirely new cancer, just like anyone else. She hasn't attained some kind of immunity.