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.
Especially considering the IFR (and it's predecessor EBR II where the experiment the parent refers to was done) are pool type reactors, meaning the reactor sits in a big pool filled with the primary coolant, there are no pipes where the primary coolant leaves the tank.
It has low power density because it's a gas cooled reactor. Gas is a much worse heat conductor than a liquid.
I'm slightly skeptical wrt gas cooled reactors. Due to the low power density, the reactor needs to be physically big. And since the gas is under high pressure, the reactor needs to be a pressure vessel. Big pressure vessel usually means expensive.
One intriguing development is to use the TRISO kind of fuel that Ultra Safe has developed, but instead use molten salt as the coolant. That allows much higher power density, and doesn't need to be pressurized either as the operating temperature is well below the boiling point of the salt at atmospheric pressure. It also avoids the problem of having the fuel and highly radioactive and corrosive fission products dissolved in the salt, as molten salt reactors usually have.
