I'm not 100% certain on whether or not this melanoma trial is powered to detect stat sig on OS benefit.
No one would ever claim a trial of 91 people was 'powered for survival'. As for what the measured HR (appox equal to ratio of medians) needs to be to be stat sig - about 0.62 or better. And to be 80 percent powered the true HR would have to be about 0.51.
As I said, no one would claim they designed a trial for an HR of 0.51.