In this ending (called Aaron ending), the planet they found was Earth. This was my first ending and I remember how confused I've got because of Phantasy Star 2 ending. It seemed to be a contradiction. In PS 2 ending, the final boss said he was from Earth and it was destroyed by greed. But I was very excited and I loved it.
As a fan, we can always give an explanation. As Revalia sugested, the black hole (wormhole) could have make the ship travel to the past. In that case, they could even be the responsible to bring Darkfallz to Earth and, in their future, it could be what made Earth to be destroyed. Then, some survivors could have tracked Algol System and fled to that system just before PS2 story.
It would be a time loop event, very common on sci-fi.