Did you get to the final paragraph of that article?
The new measurements suggest radioactive decay provides more than half of Earth's total heat, estimated at roughly 44 terawatts based on temperatures found at the bottom of deep boreholes into the planet's crust. The rest is leftover from Earth's formation or other causes yet unknown, according to the scientists involved. Some of that heat may have been trapped in Earth's molten iron core since the planet's formation, while the nuclear decay happens primarily in the crust and mantle. But with fission still pumping out so much heat, Earth is unlikely to cool—and thereby halt the collisions of continents—for hundreds of millions of years thanks to the long half-lives of some of these elements. And that means there's a lot of geothermal energy—or natural nuclear energy—to be harvested.
more than half - significantly less than all
other causes yet unknown - we're like, all, duh, and stuff
may have been - see above
primarily in the crust and mantle - not the core
hundreds of millions of years - significantly less than billions of years