That's wrong too. While geothermal heat is going down, it's doing it very slowly and has very little impact on surface temperature. Earth was on average significantly colder in the past, to the point of there being several "snowball earth" episodes when the entire surface froze over. That's no longer possible, strictly because the Sun is warmer now.

Mars was somewhat warmer before, but that was not because of geothermal, but because it used to have a thicker atmosphere, and that retained heat better.