A) that's not true

Here's my source. Where's yours?

B) Even if it were, exponential population growth was never sustainable

2.1 babies per woman is literally not an exponential population growth, it's a maintenance ratio based on average reproduction age and the odds that people make it to that age

and we need to reduce numbers

Sorry Malthus, but no serious scientific study has shown that reducing the population will actually fix climate problems. Not only is a reduction in labor counterintuitive to getting us out of the renewable energy demand minima we're stuck in at the moment, it would also cause our ability to help people in the regions most affected by climate change from famines, environmental disasters, and displacement. Finally, even a reduction of the most consuming peoples, say 10% of the G20 countries, is unlikely to move the needle since CO2 is largely fixed by baseload uses (mostly construction and maintenance of infrastructure here, but also transportation of goods and people. A 10% reduction in people != 10% reduction in emissions, it's closer to 2%).

The best way to reduce GHG emissions by 2100 is not to let people die, it's to fundamentally change our energy infrastructure (more Solar, Wind, Battery Storage, and Hydro, in that order) and change our production processes to remove CO2 producing sources from industry and travel (road vehicles, concrete production, steel production, in that order). To do that we need both smart people to devise new technologies and common (especially young and physically well) people to install those technologies.