I don't believe they are talking about malthusianism. That's a straw man. The arguement is that the sheer number of people is causing damage to the natural environment, not that we can't grow enough food to feed people.
The problem is that the problem is not the sheer number of people, it's the ratio between people and resources.
The problem is that the problem is not the sheer number of people, it's the ratio between people and resources.