Design an emitter follower to operate in the audio range (20 Hz - 20 kHz)
Win Hill et al. are not really talking about deliberately filtering the signal, rather about the requirement that the amplifier must work over that frequency range. 

As you've correctly stated, none of the parts there deliberately restricts the upper frequency range, and one could expect it to work up at least into MHz even with a jellybean BJT and a hearty 1mA of bias current. At higher frequencies (tens or hundreds of MHz), the transistor characteristics and parasitic capacitances will come into play. 

While you could add a capacitor as you suggest, it might be better to filter the signal before it is amplified. Otherwise you'll be wasting a lot of power in the transistor if it encounters a lot of high frequency input. For example, a series resistor on C1 and a capacitor across R2.