I'd describe that a little differently: I think AlphGo's choosing safer moves shows strength. In go you don't get any extra credit for winning by more; a win by 0.5 is equivalent to a win by 50.5. As such there is no incentive to fight for wins at a higher margin. If a more aggressive move is riskier or harder to read (it usually is) there is actually a disincentive to play it. I can't tell you how many games I've lost because I misread a complicated variation which I could have avoided entirely when I was ahead.