Can you link to what you are reading? I am a nuclear engineer, I suspect they are using a very loose definition of critical, one that would imply the entire universe itself is critical.
I don't think anyone knows for sure what is going on down there.
My theory is that it's both critical and quiescent, with a violent dynamic at play. Gravity will concentrate the heaviest and least stable elements as you'd expect. As that happens pockets of dense matter in close neutron proximity will approach critical and heat billions of tons of matter which then convects outwards, dispersing the mixture. Think of a lava lamp.