The reason that the whole "altruism isn't real" thing is useless is because it really tells me nothing about the world. Like, take Pippa's story about her car having a flat tyre. Why did the stranger pull over, help Pippa, refuse to take her money and leave? He was being selfish. What about the people who drove past and did nothing to help? They were also being selfish. You've told me nothing about the world and not given me any tools to understand it better.
Adopting the whole "everyone is selfish" mantra is really more telling of yourself than it is of the world around you.