I think it's more about comparison with currently available...

I think it's more about comparison with currently available treatments.
It needs to be safer and more effective (better response rates) than currently available treatments or emerging treatments. I haven't done research on what the current best treatment response rates are but as you say the very fact that these patients are in the the study would suggest failure of current treatments and that the patients are at a pretty advanced stage to begin with.
Also as a proof of concept viral RNA was found in lung cancer and bladder cancer tumours. (Not sure about prostate I'm a bit confused on that)