This is correct, but I don't think it quite answers the OP's question.

I believe they're asking why, if those cancer-specific cell surface markers are already present on the surface of malignant cells in the body, the immune system doesn't generate antibodies to them.

In other words, what's special about an mRNA vaccine and the way it presents tumor-associated antigens to the body that prompts an immune response and antibody generation, when those same antigens don't do anything by themselves when they're sitting on the surface of a cancer cell?