Meh, he's making a mathematical argument that dynamic languages are a special case of static languages. That has nothing to do with whether a language is good/productive for writing programs in.

I forget who said this, but programming language design is basically "applied psychology", and if you admit that, your language will go further. For example, the difference between Python and many similar languages is that it looks nicer and is "intuitive" to read, and that contributes a large part (perhaps the largest part) to its popularity.