mastersracer said:No, they are not estimated based on ascent times. They are from their published SRM files. The only uncertainty is rider weight. Nibali's weight is listed either at 64 of 63 kg. At 63kg, his average on that last climb would be 5.7 watts/kg.
Exactly - "listed" weight, not actual calculated mass. You don't know how much he weighed, how much his clothes weighed, how many gels he had in his pockets, how much fluid in his bottles, the inflation pressure of his tyres etc.
All of these create uncertainties and errors in the determination of speed from purely w/kg.