I guess that's a bug in the underlying simulation that needs fixing. Planetary bombardment, even "clean" ones (kinetic rather than nukes) would slowly lower the ability for a race to produce food, which would tie with the environment variable. That should result in slowly lowering birthrates, even into the negative.
Although, TLF does have artificial food production in it's tech tree, so that would allow industry to sustain a population, but that should also lower with bombardment.
Throw enough nukes or small rocks at a planet and even deep underground settlements will be exposed or at least cooked by magma. Planetary crust is only so thick after all.