Because there are two systems the next MGS game could arrive on
Three. Remember, Twin Snakes was on Gamecube.
Incidentally, all three next-gen consoles have that symbol on their 'power on' button. Not necessarily green, but then again, green has always been the symbol for 'go ahead'.
Now, if it was a RRoD or the stylized X, I'd agree it's the 360. As it is, it's just Kojima screwing with our minds. Something he likes to do a lot, I might add...