Let me offer an easy explanation: normal rates of annual progress in cancer treatment and prevention were compounded by (quite abnormal) decrease in life expectancy. Which naturally makes cancer, which correlates highly with age, less common. Basically, people are dying of meth or fentanyl before they get a chance to get cancer.
This possible explanation is explicitly addressed and debunked in the article as the reason for cancer deaths declining. As has been mentioned elsewhere in the thread, this is about immunotherapy.
