I found "A philosophy of software design" to be a well intended but somewhat frustrating book to read. It seemingly develops a theory of software architecture that is getting at some reasonable stuff, but does so without any reference _at all_ to the already rich theories for describing and modeling things. I find software design highly related to scientific theory development and modeling, and related to mathematica…
Preface: I'm likely nitpicking here; the use of "_just_" is enough for me to mostly agree with your take.
Isn't the idea that the bulk of complexity IS in the understanding of how a system works, both how it should work and how it does work? We could take the Quake Fast Inverse Square Root code, which is simple in "size" but quite complex on how it actually achieves its outcome. I'd argue it requires comments, tests, and/or clarifications to make sense of what its actually doing.
How do we measure that complexity? No idea :) But I like to believe that's why the book takes a philosophical approach to the discussion.
I agree the arguments in the book largely "make sense" to me but I found myself finding it a little hand-wavey on it actually proving its points without concrete examples. I don't recall there being any metrics or measurements on improvement either, making it a philosophical discussion to me and not a scientific exercise.