The cranking amps required to start an engine are not the same as the cranking amp rating of a battery. I don't know the amps required to start your car's engine, but if it is less than 400 amps, then it will work.
Higher cranking amps in a battery yields additional benefits. The battery can be used for more starts because less capacity is used on each start. The battery can be used for starting in colder temperatures because the reduced capacity caused by the cold weather leaves enough left over to still crank the engine. Since these things are probably not required for the jumpstarter, the manufacturer may choose to use a smaller battery
Additionally, the car's battery is expected to power loads without an alternator, so it is performing more duties than just cranking the engine. Since battery cranking amps and capacity are directly proportional in a lead acid battery, the higher capacity may be for powering other loads, giving it a higher cranking amp rating as a benefit. Since the jumpstarter is a single purpose device, the battery does not need to have such a high capacity, thus only needs to produce enough cranking amps to start a vehicle.
That jumpstarter is rated at 12 VDC, which I assume is the same as your car's battery. It's possible that if someone connects the leads in reverse, they may damage their vehicle's electrical system. However, the same would be true when using jumper cables.