He just knows that someone on HN who is not using their real name has described witnessing government fraud at some unspecified point in the past and reporting it to the head of the project. He doesn't have any information about where it occurred other than probably the United States.

He's not really in a position to act usefully on this information, so had no reason to feel any culpability for not acting. It is only an interesting question when put to people were in a position where they had to make a choice.