They're probably using fusion neutrons to kick off fission reactions. People have talked about this idea for years but I don't think anyone's attempted it until now.
By adding uranium you don't need to get net power from fusion alone, which is really hard.
The extremely high-energy neutrons from D-T fusion can fission U238, not just the scarce U235, which means you don't need enrichment and you get very little long-term waste.
By having the fusion reactor as a neutron source, you can use subcritical amounts of uranium, so there's no chance of a runaway reaction. If something goes wrong with the fusion reactor it just stops, and without the fusion neutrons the fission shuts down too.