What is the space stuff at the end of season 4 about?

In the last episode of war of underworld kirito, asuna and alice find a way back to underworld, but when they land there they are like in space and there are spaceships fighting some kind of weird eye monster in space, but then it just and ends. (for clarification i have not read the novel yet and i am going purely of of what ive heard) In the novel the next arc is moon cradle but thid arc plays during the 200 years and not after that, and the arc after that is unital ring, wich doesnt relate to underworld at all? So what was that, was that something only the anime did, or am i getting the next arcs wrong?