But he called move 79 a mistake. So did he learn from a human commentator that it was a mistake, or was that AlphaGo's assessment?

Because it is actually trivial in MCTS to reevaluate moves already taken once you have a better assessment of the positions that follow it.

I would love to hear from the team for sure, but I suspect that they have, by now, fed the game state as of move 78 into AlphaGo, had it look exhaustively at possibilities (way more than in the few minutes it took during the match), and determined that in hindsight move 79 was indeed not an optimal response.

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.