So is this thesis "don't worry about doing good, because effects can be unpredictable"?
It is indeed freeing to ignore externalities.
It is indeed freeing to ignore externalities.
Instead, let's look at impact as an indirect outcome of a ton of experimentation, evolutionary pressure and a complex system. Then the way to have impact is to experiment. So focus on doing that instead of getting stuck in figuring out how to "do good".