1. Given the lack of other constraints, humans tend to use the maximal resources available. If the world population dropped to 10 million, but those 10 million people used the bounty left behind by the 7 billion and used private jets to go everywhere, we'd still be screwed.

2. The goal is net-zero by 2050. Without any other change, that means 0 humans by 2050? Stopping having babies isn't going to get anywhere near that. For population change to have any effect on the environment, it requires a drop in population into 9 figures in a decade or three. Nuclear war is the only way to achieve that, and that's not going to be too friendly to the environment...

3. Net Zero is achievable without drastic solutions. There are lots of better options, but the backstop is carbon sequestration. At a cost of $100 - $300 per ton, that's tens of trillions for net zero. Incredibly expensive, but achievable for the modern economy. A lot easier than removing a few billion people from the planet in a few decades.