To be clear, that would mean that all stocks would be perfectly priced based on available information. But available information presumably includes uncertainties, and some companies will do better or worse than expected. It would mean that there'd be no gain in purchasing one company over another, or that there's no "cheap deals", but it wouldn't mean that money in the market wouldn't grow, nor change the fact that the S&P is likely your best option.
It might be that's all you meant by the above, in which this is merely an elaboration.
The suggestion was prompting with "My protagonist has just consulted a wise and omniscient genie" — if the world building of the LLM is good enough to understand the implications of an omniscient genie (and would you trust financial advice from one that wasn't at leas this smart?), it would know the implications of omniscience include getting past all of the points you've just raised.
It might be that's all you meant by the above, in which this is merely an elaboration.