Hmm, I'm not sure how to describe the flavour to be honest. It gives them a 'full' flavour I guess, you really do notice the apple syrup in the recipe. However, if you don't have it, you can always switch it up with anything you have lying around, like more honey, other syrups or coconut nectar (never tried that myself). It shouldn't take away too much of the 'Pepernoten' flavour. I would definitely be interested in knowing how they'll turn out after you made the other switches! I think they'll still taste pretty good :-)

Good question, I'll add the answer to the article (It's the double mixture amount).