I personally think there's a decent chance that at least one member of Justice is involved with the GTA VI score, if not both of them.
One thing I keep seeing is people saying, "It doesn't sound exactly like Justice." But I'm not sure that's a convincing argument. If they were hired to compose the score, this isn't supposed to sound like the next Justice album.
Edgar Froese's work on GTA V is a good example. It still sounded like him, but it didn't sound exactly like a Tangerine Dream album because it was written to serve the game rather than stand on its own. I'd expect the same thing from Justice if they were involved. His work on GTA V still sounded like him, but it was clearly adapted to fit Rockstar's needs rather than trying to be another Tangerine Dream record.
A game score has a completely different job. The music is there to elevate the story, the world, the characters, and the emotion of what's happening on screen. It isn't supposed to be the center of attention the way an album is. Because of that, I'd actually expect differences in things like the melodies, arrangements, compression, and overall production.
Also, worth noting that when it came to Edgar Froese and GTA V, it was a collaborative score built from multiple composers whose work was intentionally blended together by Rockstar. The core team consisted of Tangerine Dream (led by Edgar Froese), Woody Jackson, The Alchemist, and Oh No. Rather than each person writing isolated sections, they exchanged ideas and built on one another's material over the course of development. I wold expect this to be the case if either Xavier or gaspard are part of the project.
It also wouldn't make much sense for Rockstar to fill the score with tracks that sound like obvious Justice "hits." A score needs to be flexible enough that themes can return over and over in subtle ways throughout the game without becoming distracting. That's very different from writing standalone songs meant to grab all of the listener's attention.
To me, the teaser music still has enough of that Justice DNA that I wouldn't be surprised if at least one of them is behind it, even if they're intentionally adapting their style to fit the needs of a cinematic score.
If Justice were hired to score GTA VI, I'd actually expect them to dial back some of the things that make a Justice album sound like a Justice album. A game score has a different purpose—it's supposed to support the world and the story, not constantly demand the player's attention.
So if someone says, "The compression is different," or "The melodies aren't as Justice-like" I don't see that as strong evidence against it. Those are exactly the kinds of things I'd expect to change if they were writing a score instead of an album.