Frequency Domain Analysis with Transistors
Yes, the book is playing a bit fast and loose with conventions -- but because they're conventions and not hard-and-fast rules, you need to just roll with it and try to understand the author's intent.

That expression is calculated in the Laplace or Fourier domain, to find the response of the circuit to sinusoidal signals in steady-state.  So by that measure, and by "normal" convention, the author should have used \$I_d\$, \$I_{in}\$, etc.  However, the notation above is practically a convention in itself for this sort of circuit analysis so -- roll with it.