1. climate change is an existential threat to humanity
2. having one or more children is a good thing
If you believe one, you need to yield on the other. The article yields on (1) to support (2).
The thing is that (2) faces headwinds that have nothing to do with climate, namely economics. It's insanely expensive to have children. Not just the actual out-of-pocket, but also the opportunity costs, not to mention the emotional toll if the marriage becomes unstable.
Then there's the toll on children raised by people other than their parents due to the horrendous economics of both parents being at work.
So in this sense, the climate crisis as broadly defined, has a finite amount of room to run. Economics acts to cap births in developed economies. The rest of the world eventually catches up. Before you know it having a couple of kids has become a status symbol.