This argument strikes me as circular. All market participants understand the Fed wields power not just in its ability to manipulate the funds rate, but also to "jawbone" the markets in the direction it desires. If the Fed expresses concern about the future of economic growth - whether through monetary policy (keeping rates "lower for longer") or the language it uses in delivering policy decisions - this very clearly signals participants should also be concerned and feel uncertain about outcomes. If anything, I believe this effect is underestimated. So I am not convinced it is exogenous fear.