"No, the baguettes' recipes."
Translation:Non, les recettes de baguettes.
After some reading on the subject I have reached a (perhaps false) conclusion. de should be used when referring to the general rather than the specific. In this case, recipes of baguettes in general. Had it been referring to particular baguettes recipes of the baguettes then it would have been correct to use des.
In this exercise les refers to the recipes not to the baguettes. They are baguettes in general hence de.
Of course I may be completely wrong so don't necessarily take my word for it. However looked at in this way the answer given does make sense.