There’s a lot wrong with this manifesto. Let’s start with this:
“A designer does not believe in edge cases.”
The author seems to be conflating use cases with personas. Given his idealogical bent, this is not surprising. Having said that, the reality is the designer composes to a set of use cases, stories, bullet points — whatever— some of which have greater value to the business than others, or are more critical to the users (or both). The higher value items therefore presumably reflect the user’s goal or primary task(s). The lesser value items necessarily become edge cases and have lower priority in the design effort and resulting design. All of these items, high-value and low, are derived from the design team’s conception of the user population: Some users are more important than others given the business goals. A design team can’t design to everything and everyone, right? The notion that this common-sense reality of design somehow marginalizes people is nonsensical.