The text immediately following the passage you quoted reads:
for example: we can plausibly only affect a finite subset of the universe, and an infinite quantity of happiness is unchanged by the addition or subtraction of a finite amount of happiness.
This implies that the quantity of happiness in the universe stays the same after you save the drowning child. So if your reason for saving the child is to make the world a better place, you should be troubled by this implication.
That is precisely the argument that I maintain is only a problem for people who want to write philosophy textbooks, and even then one that should only take a paragraph to tidy up. It is not an issue for altruists otherwise—everyone saves the drowning child.
The text immediately following the passage you quoted reads:
This implies that the quantity of happiness in the universe stays the same after you save the drowning child. So if your reason for saving the child is to make the world a better place, you should be troubled by this implication.
That is precisely the argument that I maintain is only a problem for people who want to write philosophy textbooks, and even then one that should only take a paragraph to tidy up. It is not an issue for altruists otherwise—everyone saves the drowning child.