I'm sorry, but this is incorrect. Based both off the information in the in game manual (Menu - Help), as well as loading screen tips and actual experience with it, supply lines do in fact share food and water.
Try it out yourself. Start a new settlement, but don't plant any food or set up water. You'll see that when you have settlers there, the food and water are listed in red, and you will also get the little warning sign on the workshop page of your pip boy next to them. Now, assign a supply line. Provided you have an abundance of food produced vs the amount used, your food and water warning signs will go away, as well as turn blue when you're at your workshop menu for your new settlement.
If this wasn't the case, the settlers I assigned over to the Aiport settlement would have starved by now, since I didn't plant any crops. Instead, they do just fine and happiness contiunes to rise.