At work, I attempted to structure our 'team documentation' (ways of working, run books, meeting notes, planning, that sort of stuff) this way. It was a disaster, didn't work at all.

I agree with the author, a 'concepts' section is the big thing I find missing from the otherwise very useful 4 doc framework. This is also the most common addition that I see in open source projects, e.g. Istio and Kubernetes