Actually it isn't dumb. If you had to assign a probability to that tweet being true when you read it, then what would you say? 20%? 40%? So yes, given that there is a real probability that the markets are about to tank, that would effect your valuation instantly.
You have a point there.
If it was dumb people wouldn't be making money doing it. If information like that is only accurate 55% of the time it's still better than random (and even random would break even.)