It means that particular dc-dc that you linked to on accepts an input of 12V not 9V not 13V therefore in a car where the voltage ranges from below 12v when the battery is low or during crank and up to 14v when the engine is running, with an unregulated PSU either it will shut down because the voltage is out of range, or it will supply your motherboard with incorrect (higher) voltages.
I agree, my Opus just arrived (only 2 week or so wait). Its a quality supply and came complete with cables, including the 2x2 12v supply some ATX boards need for AGP power. Neat. Opus has a great reputation. When my replacement m/b arrives I look forward to seeing it working.