I think they would correctly point out that cancer is not a new species.
The bigger question is this: if (1) evolutionary genetic tools correctly infer the progress and history of cancer, (2) cancer regularly innovates with proteins of novel function, (3) regularly exhibits convergence at a molecular level, and (4) all the mathematical of machinery of neutral theory works so well, THEN what magically prevents all these things from being true at the species level?
This all cannot be true for cancer, but false for evolution. That is the real inconvenience here.