I think you're mistaken. The author is very careful in his use of the term growthism versus growth. He doesn't at any point explicitly define what growthism is, but that's because the whole point of the essay is to introduce the concept to a new audience, so most of the essay is given over to (an admittedly diffuse) description of what, exactly it is.

your point that the essay seems ranting, is valid, and definitely a weak point.