If it was true, couldn't you get the same effect by taking a biopsy, fragmenting the cells, and injecting them back in? Like a vaccination, in fact. Somebody must have studied that approach already.

Interested layman here: IIUC, immunotherapy is currently the holy grail for difficult-to-treat cancers like pancreatic. There are designer mRNA vaccines available that have ridiculous efficacy, but they must be tailored to each individual and so are extremely expensive (and are currently undergoing trials). mRNA COVID vaccines have been shown in some studies to increase the lifespan of pancreatic cancer patients. So, it's not hard for me to imagine that a treatment that gives the immune system a crack at learning to identify and destroy pancreatic cancer cells will boost survivability.

Part of the freak-out about the Trump admin's attacking of scientific research (including, especially, of mRNA research) earlier in the year is that it threatened these trials.