I've found that many of the general principles for application "user interface" design also apply to code design itself. The code and config info is the programmer "user interface" into the application model. Similarities include:
I'd certainly agree that LanguageDesign (including API Design, as per ApiIsLanguage) needs the same principles as UserInterface. The 'user' is another programmer. And, vice versa, users (or at least an important subset) are often programmers, which leads to user-defined scripts and plugins and configurations and other extensions to applications. That said, the "ALL" in AllProgrammingIsUserInterface seems a stretch. Glue code and other sorts of code that you don't intend to share with anyone, especially if it's a one-off script, doesn't seem to qualify so readily as UserInterface...