Your instincts are correct. The approach in the paper is more complicated than this. Here is the abstract:

Abstract The success of cancer immunotherapies is predicated on the targeting of highly expressed neoepitopes, which preferentially favours malignancies with high mutational burden. Here we show that early responses by type-I interferons mediate the success of immune checkpoint inhibitors as well as epitope spreading in poorly immunogenic tumours and that these interferon responses can be enhanced via systemic administration of lipid particles loaded with RNA coding for tumour-unspecific antigens. In mice, the immune responses of tumours sensitive to checkpoint inhibitors were transferable to resistant tumours and resulted in heightened immunity with antigenic spreading that protected the animals from tumour rechallenge. Our findings show that the resistance of tumours to immunotherapy is dictated by the absence of a damage response, which can be restored by boosting early type-I interferon responses to enable epitope spreading and self-amplifying responses in treatment-refractory tumours.