Let's consider a recent example. The massive Fed/Treasury coordinated $2T+ bailout from last week. This thing came as a surprise to most us. Prior to this we saw 10 days of "weird" bullish activity. Market was going up on incredibly bad news, some banks such as Goldman advised high-end clients to buy stocks for unknown reason. And then the news came. Later it was revealed that this SPV had wide congressional support. So 1000s of officials from the White House, Fed, Treasury dept and Congress were sitting on this info for 2 weeks. Now add their staff that worked on getting this done, lawyers, accountants, advisors, regulators, oh and BlackRock who executed the deal. There is exactly zero chance this piece of info wasn't leaked to all high end clients in the banks at least a week in advance.
I don't doubt it was leaked. But even with that information, unprecedented Fed support was speculated for some time but crystalized on March 3rd when Fed lowered rates 0.5%. the market still went down since then. Maybe it didn't go down as much as it would have had the Fed not intervened, but that's difficult to profit from. Market recovered a bit and you can point to another Fed policy in hindsight and say this caused the rebound.
