A part missing in your analysis is that time is involved in the decisions, and that time is continuous, and that time is very important
Is time really continuous in this case? I have absolutely no idea how it's implemented in practice, but would assume that stock exchange computers use some sort of game loop at a consistent rate.
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.