Why is the Fourier Series a special case of the Fourier Transform and not the other way around?
That's more of a philosophical question. It makes no practical difference if you choose one answer over the other.
Personally, I would agree with the author. I find it  mathematically a lot easier to derive the Fourier Series from the Fourier Transform than the other way around. It also makes more sense to me to say &quot;periodic signals are a subset for all signals&quot; as compared to &quot;all signals are periodic, some just have a period of infinity&quot;. That kind of defeats the purpose of having periodicity as a specials case.