I'm suggesting that, for instance, a 15 index player from a low slope course is a much less skilled player than a 15 index player from a high slope course.
This could be tested very easily, could it not? Create cohorts of 15 index players from high slope and low slope courses. Then track their scores at other courses and see if they play to their handicaps.
First of all, neither group will play to their handicaps but once every 4 or 5 rounds, regardless of the slope. If they did, that wouldn’t be their index anymore. One round is a very small part of the picture.
Secondly, to have a meaningful test, players would have to play at least 20 rounds at each course with very different slopes, and that’s putting aside questions of weather and course conditions. While I’m sure there are guys out there who play 20 rounds per year at one away course, I don’t think I know any; I certainly don’t, and I play a TON of rounds per year. How exactly would you get a cohort of guys at a low slope course to play 20 rounds at a high slope course, and vice versa?