Demis tweeted that while the game was still ongoing. Anyway I'm pretty sure what happened; I have actually implemented MCTS myself and know that you can trivia

Demis tweeted that while the game was still ongoing. Anyway I'm pretty sure what happened; I have actually implemented MCTS myself and know that you can trivially update evaluations of old nodes in MCTS as the game continues and you get a better estimate of the line of play actually taken, although you wouldn't normally have a reason to do so. Basically, it can see in hindsight that a move it took was bad, but without additional calculation you wouldn't know whether the alternatives were any better, or also are worse than estimated at the time.