Earlier quoted context omitted.
Positions with 99% win percentage are not decisive for GM vs non-GM rating.
From the paper: If Stockfish detects a mate-in-k (e.g., 3 or 5) it outputs k and not a centipawn score. We map all such outputs to the maximal value bin (i.e., a win percentage of 100%). Similarly, in a very strong position, several actions may end up in the maximum value bin. Thus, across time-steps this can lead to our agent playing somewhat randomly, rather than committing to one plan that finishes the game quickl…
Yeah, they didn't use Stockfish for the lols.
They create a search-less engine for chess. And then used a search engine to pay a small minority of the game.