Sponge tool Question

That is probably old book. There are many other easier ways to make subject stand out on black and white background. Use selection tool then apply Black and White adjustment or Hue/Saturation > remove Saturation (negative number). You can also try to desaturate specific color using Hue/saturation > choose specific color from the top drop-down menu.

The Sponge tool can be used to either saturate or desaturate colors. Saturate removes gray making the colors brighter and purer. Desaturate adds gray. If you desaturate fully, the colors will turn gray. that is what is happening in the second picture, the lower one.