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. This thread is worse than the original Carnival of Stupid.
I don't even play MGS games and it's apparent that Jett here is on the money. C'mon, Moby Dick Studios? Chasing the white whale? It seems like a pretty obvious effort to drum up excitement among fans by exploiting their existing schema regarding game reveals to lure them into a chase of mystery and suspense, at the end of which will be MGS5 (or whatever it's called then).