I think that's the difference though (not to besmirch indie games)."major" is a pretty narrow goalpost here. There are oodles of indie games starring trans characters made by trans people.
This is a major title published by Microsoft as an exclusive with a lot of money and lots of development talent behind it.
It's kind of the same reason Black Panther was a big deal for the MCU. Did black superheroes exist on the big screen before? Yeah, of course. But there was always caveats before - usually not in a leading role (Falcon, Rhodey, Cyborg, etc.), or they were R-rated (Blade) or they were... whatever Blank Man and Meteor Man were, etc.
This is a HUGE deal for the entire gaming industry because people do look to companies like Microsoft and what they invest in and promote, so having Microsoft and Dontnod do this game and have a major trans hero right there on the front of the box is a massive thing.