The S&P estimate is exactly that, an estimate. It's likely not spot on, but it's likely in the ballpark. The faith in the country dipped even further while people watched an idiot from Texas drag it into the ground for half a month and nobody being capable of stopping him. The shutdown hurt our reputation, it hurt our economy, and it was a very fitting display of how inept at playing like adults we currently are.
The projected amount of money lost during the government shutdown is a veritable estimate. I would personally like a more extensive breakdown of just how this number was calculated though. They should also further state that these numbers are possible estimations, and therefore not concrete stats as to how much we actually did lose. They should also state how this was "taken out of the economy"; was it in lost sales? Hourly wages not earned because of shutdown? They should be more specific.