The MLD Superlight Bivy is not a waterproof bivy, therefore, to specifically address your question: The MLD Superlight Bivy may not keep your bag/quilt dry if your backpack goes into a stream or if you encounter hours of rain.
That is what a dry bag is for. Be it a dedicated dry bag or one of the plastic trash compactor bags (just make sure you learn how to close it good enough to prevent water entering it, should your backpack go under while crossing a river) that people like to use these days.
As a SUL/XUL hiker, a dedicated 15 gram (half an ounce) bonded cuben fiber stuff sack that is 100% devoted to keep my sleeping bag and down jacket dry - no matter what - is 15 grams worth having. Your sleeping bag is arguably the most important item in a SUL/XUL backpack, do what you have to in order to allow it to do its job when it comes time to needing it.