The article mentions the pebble can handle 1600 C, and then they say it's meltdown proof. How is it meltdown proof? I assume if the rocket core runs nominally

The article mentions the pebble can handle 1600 C, and then they say it's meltdown proof. How is it meltdown proof? I assume if the rocket core runs nominally at 1000 C, if for some reason the propellant flow is lessened, it would heat up to 2000 C and just melt? Also, melting changes geometry, potentially to the direction of more nuclear reactions, meaning runaway. This has always been a problem with gas cooled particle bed reactors. Now, maybe there is a mechanism that prevents meltdown, it just isn't mentioned in the article.