It's getting more and more apparent that AlphaGo is probably in a league of its own. When Lee Sedol managed to put forth a decent attack against the machine, he just succeeded in revealing more strength than had thus far been seen in AlphaGo's play.
Why not before? Because AlphaGo hadn't needed to exhibit its full skill to secure a win, yet. Not in a way that a human would viscerally notice, anyway. Sure, a weird move here, a seeming mistake there, all adding up to victory in the end.
Have we now seen the full depth of AlphaGo's power? Doubtful; it would be quite the coincidence. At this point, the interesting question becomes, how much handicap would AlphaGo need to give the top Go professionals for them to have a decent chance.
(There are more interesting metaquestions of the more general implications, of course.)
"It was the first time we’d seen AlphaGo forced to manage a weak group within its opponent’s sphere of influence. Perhaps this would prove to be a weakness?
This, however, was where things began to get scary." [...]
"Lee seemed to be playing well, but somehow the computer was playing even better.
In forcing AlphaGo to withstand a very severe, one-sided attack, Lee revealed its hitherto undetected power." [...]
"By move 32, it was unclear who was attacking whom, and by 48 Lee was desperately fending off White’s powerful counter-attack." [...]
"After being compelled to flex its muscles for a short time and gaining the upper hand, AlphaGo began to play leisurely moves.
By now, most observers know that this is a feature of the ruthlessly efficient algorithm which guides AlphaGo’s play.
Unlike humans, AlphaGo doesn’t try to maximize its advantage. Its only concern is its probability of winning.
The machine is content to win by half a point, as long as it is following the most certain path to success.
So when AlphaGo plays a slack looking move, we may regard it as a mistake, but perhaps it is more accurately viewed as a declaration of victory?"