During my university studies, I took courses in electro-accoustic music composition. Significant amounts of time dealt with synthesis and signal processing because those were critical elements in these kinds of compositions.
It's absolutely different than composition for traditional instruments in this regard because the sounds you are using to compose with are being created by the composer and much as are the notes, rhythms, and structure of the composition.
So for me, the title makes perfect sense.
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).