Peanut Butter and Jelly

From Mankiw's Principles of Macroeconomics:

The bit about peanut butter and jelly may be true of the market as a whole, but personally I never use peanut butter and jelly together. I like peanut-butter-on-toast and jelly-on-toast, but I am immune to the appeal of a PB&J. If anything, I see peanut butter and jelly as weak substitutes.

This is not the first time that I've found a statement about spreads in an Economics textbook which runs counter to my own preferences.