Preventing cancer seems like a fool's based on my understanding of the disease. You have cancerous cells in your body every single day. Your immune system kills them all off. It's like saying you want to prevent all bacteria from entering your body. It's just not realistic (at least, not right now).
He gave a couple of examples. Another one is aggressively treating GERD so that it doesn't devolve into various conditions that lead to stomach or esophageal cancer. It's possible something like this could treat pre-cancerous things too, like Barrett’s esophagus.
Replying to a post noting a 90% reduction in cancer thanks to the HPV vaccine with "preventing cancer seems like a fool's errand" is pretty amazing. Would you care to expand on that?