In one sense, I can understand why they would choose to use Stockfish in mate-in-N positions. The fact that the model can't distinguish between mate in 5 and mate in 3 is an implementation detail. Since the vast majority of positions are not known to be wins or draws, it's still an interesting finding.

However, in reality all positions are actually wins (for black or white) or draws. One reason they gave for why stockfish is needed to finish the game is because their evaluation function is imperfect, which is also an notable result.