alt Hacker News"Monkey wrench" is a word already found in the dictionary, so it wouldn't be a useful example. It already met the bar.
The article is questioning why some words don't meet the bar for inclusion in the dictionary. The word "boiling water" is one such word that it sees as being on the fence. The comments here demonstrate exactly why it is on the fence, but it remains unclear exactly what would be necessary for it to tip towards inclusion.
It is objectively a phrase, and not a word, because you can substitute the water for literally any other liquid and form a perfectly coherent phrase. boiling oil, boiling syrup, boiling coca cola. "Boiling" in this context is just a participial adjective, modifying the noun "water". If "boiling water" is a word, so are "six men", "good idea", "large rock", "7 year old boy", "Californian trees", "metallic flooring", etc.
Better yet, you can take advantage of English's adjective ordering to demonstrate this point. Would I describe the water I'm currently boiling for the purpose of cooking "cooking boiling water", or "boiling cooking water". Since purpose tends to be the last adjective we use, any native speaker would choose the later.