If you recall from the season 3 episode "Jack and the Travelling Creatures," this is the opening to Jack's epic duel with the Guardian. At the end of Jack's duel with the Guardian, it was prophesied that Jack would return to face and defeat Aku -- older, bearded and crowned as a warrior king, holding his sword -- using the time portal in the distant future. Since Jack's gotten a makeover this season, that future might no longer be in play or it could just be retconned into a new future. For my money, it still is in play, but in a tweaked form. Otherwise, why revisit the place where he fought the Guardian at all?