I think you missed the connection. Gates did not want to be the richest person in the world any more. Giving that money to charity was a socially acceptable way of doing so. Thus, it may have been a selfish action with a net positive outcome.

The pimps homeless shelter might also be a net positive impact. It seems far more selfish.

Alternatively it may have started Gates down a different path which changed his mindset. I can't know as this is all internal. The point is some people feel the internal reasoning is just as important as the outcome.

PS: This is one of the reasons the law cares about internal reasoning not just outcomes.