The first sentence of the foreword brings to the point, what the book is about:
"The Theory and Technique of Electronic Music is a uniquely complete source of information for the computer synthesis of rich and interesting musical timbres."
Whereas tools like Max Mathews' (btw. the author of the foreword) MUSIC programs and their successors clearly separate music composition and instrument building (i.e. sound synthesis), later tools like Max, PD or SuperCollider are blurring this difference. Nevertheless the difference is still maintained by all institutions where electronic music is studied and performed (e.g. IRCAM).
On the matter of institutions: IRCAM is the paradigmatic example of composer / technologist role demarcation, but I would question whether this extreme position "is still maintained by all institutions" -- it certainly was not at my alma mater and I doubt at UCSD either. As you say, Max (coincidentally a product of Miller Puckette and IRCAM) and it's more recent ilk have empowered composers to independently build their own instruments and this practice has been ongoing within the academy for at least 35 years now.
> "The Theory and Technique of Electronic Music is a uniquely complete source of information for the computer synthesis of rich and interesting musical timbres."
It's really a great book, but it is far from "complete" as it omits some very important synthesis techniques - most notably granular synthesis and physical modeling! To be fair, no single book would be able to cover the entire spectrum of electronic sound synthesis. The second edition of "The Computer Music Tutorial" by Curtis Roads (https://mitpress.mit.edu/9780262044912/the-computer-music-tu...) comes close, but it is a massive book with over 1200 pages and took literally decades to write. (The second edition has been published 27 years after the first edition!)
What I find really cool about Miller's book is that all examples are written in Pd so anyone can try them out and experiment further.