Indeed that's a weird thing to say, especially since AFAIK AlphaGo is for (very large) discrete search spaces (where Monte Carlo Tree Search would be used), & equity trading doesn't strike me as a discrete search space.. is it ?
The bigger difference in equity trading is that it's a "game" of hidden information with asymmetric and maybe unknown payoffs. Of course, computers are pretty good at trading stocks, too, but I can say confidently AlphaZero won't be the most useful approach.