A few points,

I think the article is talking about the temperature at the surface of the Earth, which is only a tiny fraction of the volume of the Earth. So when one is talking about the temperature of the Earth, or the rate of cooling, it makes a difference if one is considering the surface or the interior.

Second, it isn't just a linear cooling of the Earth from formation to the present. The Earth also generates heat from several sources, including radioactive decay, tidal forces, gravitational differentiation (not sure about the right word, but I mean dense materials settling toward the core). So the interior of the Earth cools slower than you would expect.

__________________
"I'm as accurate as any psychic. And I'm a cartoon!" -- Squidward

"Arrrgh, the laws of physics be a harsh mistress!" -- Bender