Is the effect of capacitors and inductors directly proportional to frequency?
I believe the misunderstanding here is they wording only. The magnitude of the impedance of a capacitor goes down as frequency increase, as you have stated. 

However the book's use of the word "effect" is the deviation from the DC steady state expected value. A capacitor at DC is an open circuit and the higher you raise the frequency the closer it looks like to a short circuit. An inductor, however,  looks like a short circuit in steady state DC but as the frequency increases it looks more and more like an open circuit.

These comments are only about the magnitude of the impedance.