He's correct. Since Space-Time began with the Big Bang, the Cosmos is all there ever was, is or will be.
Was it Flatland that had two-dimensional creatures trying to comprehend a cone that was penetrating their plane? To them it began as a point then a circle that grew larger and large then suddenly disappeared when the cone passed through like the middle one below. They couldn't comprehend the three dimensional object passing through their flat world.
Similarly, we see everything in only three dimensions and can't comprehend fourth or greater ones. The "before" the Big Bang might just be a greater dimension.