You don’t. And he doesn’t claim that, does he?

The author of the article has previously often advocated user testing, not trusting your gut.

Research, with 'unusual' icons, shows that some icons are more confusing than others. Specifically, icons that have too few or too many details. Now you could argue that a certain amount of detail has become the culturally entrenched norm and that such research still doesn't prove a thing, but then the burden of proof shifts to you, to back that statement up with an experiment. After all, it also simply makes sense: with too few details, you need to choose from all possible details you could add to make sense out of the symbol. On the other hand, with too many details, you need to choose from all possible details you could remove, before the main point stands out. You're just trying to minimize the amount people have to think.

There's no such thing as 'intuitive.' An intuitive design just means that it fits the user's mental model well, which is just a broader way of saying 'familiar.'

The only "intuitive" interface is the nipple. After that it's all learned. -- Bruce Ediger

With that said, we all know an intuitive interface when we see one. It's just too difficult to separate an enormously complex and highly context-sensitive technical process that we've learned from some kind of ideal optimum. Saying a design is "intuitive" is analogous to saying one Turing complete language is "better" than another.

Ignore the code is a pretty good design blog in general (http://ignorethecode.net/blog/)