"Well-defined value" was used there in the sence of "universally valid for everyone that uses them". (Which does not exclude that there are people that do not use them at all, because they have better means for judging positions. Stockfish no longer uses piece values... It evaluates positions entirely through means of a trained neural net.) If that would be the case, it would not be of any special interest to specifically investigate their value for high-rated players; any reasonable player would do. I already said it was not clear to me what exactly you wanted to say there, but I perceive this interest in high ratings as somewhat inconsistent. Either it would be the same as always, and thus not specially interesting, or the piece values would not be universal but dependent on rating, and the whole issue of piece values would not be very relevant. It seems there is no gain either way, so why bother?
"Well-defined value" was used there in the sence of "universally valid for everyone that uses them". (Which does not exclude that there are people that do not use them at all, because they have better means for judging positions. Stockfish no longer uses piece values... It evaluates positions entirely through means of a trained neural net.) If that would be the case, it would not be of any special interest to specifically investigate their value for high-rated players; any reasonable player would do. I already said it was not clear to me what exactly you wanted to say there, but I perceive this interest in high ratings as somewhat inconsistent. Either it would be the same as always, and thus not specially interesting, or the piece values would not be universal but dependent on rating, and the whole issue of piece values would not be very relevant. It seems there is no gain either way, so why bother?