I know that. So far there's no indication whether the Overlord is alive or dead, but I don't think he would let the Floor Guardians kill each other.
So I assume something's happened to him or he might already be dead. Of course, all this happens without MC knowing since he's been away from the Tower for who knows how long.
No one in the comments has said it...... but I took it as this was the future (see you in 100 years senpai) and this new kid is likely the mid-boss reincarnated 100 years later.
Also I think the one that beat him was from the tower and not from the cave which is why no one reading recognises her.