Your last point is true but they weren't measuring learning rate, they were measuring cognitive performance. I don't think chess was chosen because it's the be

Your last point is true but they weren't measuring learning rate, they were measuring cognitive performance. I don't think chess was chosen because it's the best proxy, I think chess was chosen because of the sample size and data available about players and games and an almost quantitative model of performance during each game (optimal moves). Poker would also be valid or sudoku or many others but there's usually a certain amount of chance involved in most other games and the impact of that chance is hard to estimate. I appreciated them focusing on chess exactly because of the third point you mentioned. It's a fairly even playing field in terms of learning opportunities.