The article touches on that:

> But just four days ago, mxdys and another contributor known as Racheline discovered a barrier for BB(6) that seems insurmountable: a six-rule machine whose halting problem resembles a famously intractable math problem called the Collatz conjecture. Connections between Turing machines and the Collatz conjecture date back to a 1993 paper by the mathematician Pascal Michel, but the newly discovered machine, dubbed “Antihydra,” is the smallest one that appears unsolvable without a conceptual breakthrough in mathematics.