Universe -- All matter and energy, including the earth, the galaxies, and the contents of intergalactic space, regarded as a whole.That is its primary definition, "all there is", and by extension from laws of conservation of matter and energy, "all there ever will be".
It isn't philosophical; it's definitional.
I don’t believe that I am putting words in his mouth. Sagan was a self-described, famous atheist. I observed him make this statement on his show “Cosmos”, years ago.
He was tying to make a point concerning the origins of the universe, not simply offering a definition of a word. I think most people have a good enough grasp on what is meant when somebody refers to “the universe” that they don’t need it to be explained to them.
He also made the statement that the universe “exploded into being”. Coming from an individual that assumes a purely materialistic reality, that is one of the most unscientific expressions of faith that I have ever heard.
Sagan may have been a poor philosopher, and can be forgiven his imprecise statements, but that is all the more reason for him to have limited himself to his area of expertise, and to not attempt to leverage his reputation to promote his flawed view of reality.