I believe they talk about the computational task described in [1] (published in 2016). Note that this is somewhat contrived: it's a task defined specifically so that it is very difficult for classical computers and relatively easy for quantum computers. It is also defined in a way that ensures that a quantum computer necessary to demonstrate quantum supremacy is relatively small in terms of the number of qubits.
The task is that of generating output samples from pseudo-random quantum circuits. The paper shows that the task has exponential computational complexity on classical computers and that it can be used to demonstrate quantum supremacy using a quantum computer with approximately 50 qubits.
Now, the IEEE Spectrum article indicates that the team has now built a 49-qubit quantum computer and plan to use it to demonstrate quantum supremacy.
EDIT: Replaced direct pdf link with the article page link.