I agree: the argument given here doesn't address whether existential risk charities are likely to be helpful or actively harmful. The fourth paragraph of the conclusion and various caveats like "basically competent" were meant to limit the scope of the discussion to only those whose effects were mostly positive rather than negative. Carl Shulman suggested in a feedback comment that one could set up an explicit model where one multiplies (1) a normal variable centered on zero, or with substantial mass below zero, intended to describe uncertainty about whether the charity has mostly positive or mostly negative effects, with (2) a thicker-tailed and always positive variable describing uncertainty about the scale the charity is operating on.