True, a single data point can't give you knowledge of regression effects. In the context of the original problem, Kahneman assumed that you had access to the average score of all the golfers on the first day.
I played a video game for the first time yesterday. My score was 39700, and higher scores are better than lower ones. What's your best guess for my score the next time I play it? (The answer is some number higher than 39700, because I'm no longer an absolute beginner.)
I'm not sure it's true that the answer is higher than 39700, in this case. It depends on if you have knowledge of how people generally improve, and if your score is higher than average for an absolute beginner. Since unknown factors could adjust the score either up or down, I would probably just guess that it will be the same the next day.
The existence of factors which could adjust the score either up or down does not indicate which factors dominate. In this case, you have no information which suggests that 39700 is either above or below the median, and therefore these two cases must be assigned equal probability - canceling out any "regression to the mean" effects you could have predicted. Similar arguments apply to other effects which change the score.
Here's the new thread for posting quotes, with the usual rules: