I think the assumption is that he's talking about people changing their diets. Can't / probably shouldn't advocate for parents getting rid of their children.

That said, there are probably more direct actions one can take for our ecology that eclipse consumer choices.

That's a lot of absolutism for someone not specifying a defining context.

No definition of "help", "the planet" or "diet" means it's wide open to contra-examples.

I feel like not having kids kinda defeats the purpose of saving the planet

Number one is rich kids.