I agree with you, the earlier post says people only do things out of self-interest, and I was surprised that nobody else had disagreed or mentioned it’s an unfortunate way to see the world.
I think that mindset is held by most people, most of the time. But not all people at all times. If you're in that mindset, however, the mindset itself may taint your perception of how often others are in the same mindset, which I suspect (hope?) may be the case for the person who wrote the original statement. I suspect (hope?) that person has done at least one selfless ethical/nice/kind thing at some point in their lifetime.
That bar is way too low.