What does "sustainable equilibrium" mean?

What are you balancing if not human growth? And how do you plan to do that?

Doesn't it kind of work itself out?

If it’s unsustainable population growth there will eventually be some catastrophic environmental effects and the population will be reduced to a sustainable level. Of course living during that reduction could be a rather unpleasant experience but it probably also won’t last too long.

If it’s already sustainable everything is fine.