Tetsuya Nomura has revealed that the outfits for the main trio in Stranger of Paradise Final Fantasy Origin hint that they are "not of that world".
In an interview with Famitsu Nomura was asked about the origins of the main trio and commented on the fact that they don't look how you'd imagine traditional Final Fantasy characters to look.
Nomura said, "if you’re familiar with the premise of Final Fantasy 1, you might be able to pick up some clues from the designs of Jack and the others. The initial outfits for Jack and his allies are nothing more than a hint to show that they’re not of that world. They don’t represent anything about their character. Like other games where you can change your equipment, this is simply their plain state. Perhaps."
Although Nomura is being pretty cagey towards the end of his statement, it's made pretty clear that the outfits look so strange and modern because the characters don't originate from that world. Fans have been pointing out the weirdly modern clothing and strange tone since the game's reveal, but it's seeming more purposeful now.
The interview also revealed the meaning behind the name "Stranger of Paradise", and it again ties into the theory that these characters aren't from here. Nomura said, "Jack and his allies are also “strangers”. Who are Jack and his allies? Are they the Warriors of Light? These questions lie at the heart of the story, and this idea of “strangers” is central to the plot".
The prevailing theory is that Stranger of Paradise is an isekai story, which generally features characters being transported from their world to somewhere brand new. In this case, it looks like Jack and his friends have been transported from modern times into the world of Final Fantasy 1. It's still not clear why Jack has such a grudge for Chaos, but that'll likely become clear when the game releases.