What? No, they do not "code to the metal". GTA3 was produced with Renderware, and GTA 4/5/O were made with a multiplatform engine. (And plenty of PS2 games did what GTA3 did, some better than GTA3 or even its sequels, albeit arguably none put as complete and advanced a package together in one single product.)
Rockstar's impressive work comes in large part from money and manpower. They can get tons of talent in to spend hours producing the fine details and subtle nuances that make their games robust, they can let engineers dedicate time to technical work small and large in order to complete the simulation (I'm having a hard time thinking of any real technical innovation only done or even done first by Rockstar, it's all stuff that's out there but few studios have the time and staff to put it all in one project and/or bake it on all levels needed to avoid failure points,) and they can spend an extended period of time making all that content. GTA6 is less a reinvention of this gen's capabilities than it is a culmination of everything these systems made possible if enough hard work and hard time to devoted to a game.
Yes, they also have some of the best tech workers in the business, and those people will blow minds with what they make these PlayStations and Xboxes do. But they're not reinventing the wheel; they're setting a new high water mark.