Searching thedailywtf.com for the term "proprietary language" comes up with some fun stories (MUMPS, Labview, etc).
Having taken a compiler design course in college which involved attempting to create a simple proprietary language, I can attest that attempting to write a programming language will result in bad far more often than good. The halls of any computer science program are littered with terrible language experiments that barely function, mine included.
To extend the chef analogy the author uses; a good chef may be able to make a palatable dish out of rotten meat (isn't that what curry was invented for?), but the meat is still rotten.
I'm a blub programmer, but I've used a number of languages over the years - and not sequentially, either, so I don't think my observations are due to a changing overall experience level - and in my experience, it's unambiguously a lot easier to be productive in some languages than in others.
Missing from the author's initial analogy - "Give an experienced cook a dull knife. The blade slips and he cuts himself" - is the fact that an experienced cook will know the blade is dull and respond accordingly.