DL;DR: the current 'flat design' fad is a reaction to skeuomorphism overuse and abuse. Flat design isn't really the solution either, since close adherence can hinder intuitiveness. Therefore, a compromise the author calls "post flat design" is a fusion between the two styles with a focus on content and intuitiveness rather than implementation.

Personally, I don't know if "post flat design" is the solution, but I'm sick of flat design. I prefer skeuomorphism, but agree with the author that it has been overdone: I don't need a photo-realistic leather binder for my note-taking app, for instance.