Those quotes have details behind them. Such as that sales where mostly all shifted in from the future, less cars where sold overall (hurting the industry), the fuel economy savings resulted in mere single-digit days of gasoline saved over the lifetime of the vehicles sold, and a bunch of other not so great things.
Either way, the Brookings report ends with this quote - "In the event of a future economic recession, we would not recommend repeating the CARS program."
I think I'll trust Brookings.