It's a non sequitur designed to degrade the conversation and it implies that having reservations about constant growth within the current paradigm is somehow akin to desiring large scale mass killing?
Bizarre and troubling.
A political movement to “decrease the Earth’s population” is extremely bizarre and troubling, and deserves being called out for what it is.
Humans are not some unique, special animal. We're a very invasive, ecology modifying organism. We already have rules on where humans can and cannot live, where resource extraction can happen and how much. It may be uncomfortable to consider, but yes, we absolutely should manage our own population so we don't fuck up our one, and only, spaceship that's flying through a universe that is very hostile for us.
Metastatic growth is simply unsustainable. Running our civilization like a capitalistic business ignoring externalities and borrowing against the future that needs constant growth to sustain isn't a great idea. That will crash.