I guess there's a point where that's arguably true. But it's not correct universally, and it's certainly not true if you look at earth and the sun.

The heat generated within the earth comes from nuclear decay and the energy radiates out from the planet. It would do that no matter what. Humans can tap into it and it would have the same lifespan.

If humans somehow started extracting the heat leftover from the planets formation, then yeah, that is exhaustible. There's about 100,000,000,000 times our total annual energy (not electricity) use in there. Planet only has 5 billion years left, and only about 1 billion livable years.

That time gets considerably shorter if we start pumping that energy to the surface, because we cook the surface with the waste heat.

So even assuming it was technologically possible to access the heat there, you could not draw enough to exhaust it in the time frame you have remaining to do so without killing the biosphere.