Every human voice has a different sound signature. I seriously doubt that any of the tenors mentionned would "measure" the same. I am not quite sure what type of measurement is implied or suggested though. Whatever note they sing (assuming they hit it just right, which they should on any given day)would have the same fundamental, but the harmonics would be different The harmonics are what gives each voice its character, its timbre. The harmonics would not measure the same. How this would help any of these fine singers perform is highly dubious. How any such measurement would increase the appreciation of their art by the listener is equally suspect. I hesitate to say the "tenor" argument is specious in this context, and leave it up to others to decide. If the example was between two guitars, at least the makers could attempt to duplicate the sound of a highly prized instrument on the basis of their findings, but since we can't manufacture singers the example is a bit wonky. No two guitars of the same maker would, if properly scrutinized, measure exactly the same anyway. Audiophiles might learn something by hearing the new modeling amps though. Ne less a guitar player than Buddy Guy felt that the new Fender could reproduce the tone of a vintage Fender Bassman. I wonder how Fender found the way to have digital circuitry mimic one of its own icons?
