The article was light on details. From what I can gather, the core idea[1] relies on the fact that the fusion reaction releases lots of fast neutrons. In a traditional fusion reactor they are stopped by a so-called blanket surrounding the reactor, and in turn heat the blanket. This is how you get power out of the thing.

For a hybrid reactor, you make the blanket out of uranium. The fusion-based neutrons can then trigger fission in the blanket, causing a lot of additional heating.

Found this[2] article from a few years back which goes a bit into the pros and cons of fusion-fission hybrids, in the context of another company pursuing a hybrid reactor design.

One positive aspect is that you can "burn" non-fissile uranium isotopes, so there's no need for the expensive enrichment process that a traditional fusion reactor requires. And, since fusion is not a self-sustaining process, using a non-critical blanket material means there's no chance of a run-away process.

[1]: https://en.wikipedia.org/wiki/Nuclear_fusion%E2%80%93fission...

[2]: https://www.power-technology.com/features/featurefusion-fiss...