Seems like their claim to "Ultra Safe" is actually merited based on a first principles look at the power density. They claim 1.24 W/cm^3 versus 20-40 W/cm^3 in normal reactors. The high power densities mean you have to be able to cool the reactor, even when you shut down the reactor because reactors take a long time to actually turn off. With such low power density, it should be no problem to cool down the reactor. It'll just cool down naturally, like a hot pot. In fact the metric to look at would be power per core surface area.