Yeah it is, KojiPro doesn't have the resources to develop two games of this kind at once. Hell, they're "outsourcing" multiplayer to their new LA studio. I see some people jumping through hoops for absolutely no reason trying to find reasons why it would be two games. I saw someone saying that "Ground Zeroes" will be a small, downloadable game. Others that it will be a prologue sequence to the full game. The next Metal Gear game was described as a massive open world game taking place in several locations around the world. It's not small, it's not a spinoff, it's fucking MGS5, from the start it has always been that. And this Phantom Pain trailer is part of it. It uses the same freaking assets FFS.