A dog's body basically treats xylitol like ULTRA MEGA SUGAR! This causes the pancreas to freak out and send out loads of insulin which crashes the dog's blood sugar and causes it to become hypoglycemic, so can lead to seizures and going into a coma. Can also cause liver damage - nasty stuff!
I never knew sweetened peanut butter was a thing until I started seeing people sharing photos about how PB can be toxic to dogs - went and had a look at the supermarket and couldn't find a single sweetened PB here! There is a shop that stocks American food which has PB which has sweeteners in it though - why do y'all want sugary PB?
Any sweet food could potentially have xylitol added, and a few other things you may not suspect like toothpastes and other oral care stuff (gum!), sauces, drink powders and even some beauty products.