Nobody believes that markets are strong-form efficient. There is and always will be differences in information between players in the market. It's just a useful way to talk and think about how a market with perfect information would behave. That's why semi-strong and weak form efficiency are usually taught in the same lecture :)
> Doesn't economic theory say that you can't play the market?
That depends which theory; under most that can be taken as even semi-plausibly as models of the real world, no, unless you assert some simplifying assumptions (which then make them no longer semi-plausible models of the real world).
The simplifying assumptions are common in intro-level Econ classes, for the same reason that, e.g., assuming the absence of friction is common in many parts of intro-level Physics classes.
The thing is that we don't have as many people in the public media trying to sell policy using explanations of physics that leverage the fact that most people that know anything about the subject do so through hazily-remembered intro-level classes as we do for economics...
That is not economics in the macro/micro sense, that is more financial/ financial markets theory. It really is a different branch of thinking than what economists deal with.
