In this instance, I don't think the reversal test will establish much. I imagine that most who hold a belief similar to lucidian believe that there is a population homeostasis, or at least upper bound, which is determined by Earth's resources' ability to accommodate human existence.

If this homeostasis or upper bound is exceeded, either Earth's resources' ability to accommodate human existence must be improved, or humans must be killed. I imagine those I previously identified would favor the former.

The former could be achieved by ceasing pollution, disbanding mass cattle and feces pits, and investing in research investigating ways to remove airbourne methane as well reducing existing environmental damage. This route may prove more laborious than genocide or random en masse killings, but preferable nonetheless.