Actually all the signs of that teaser point to it being a sequel, not a prequel.
1. The girl is an older versions of the Adam girls from the original Bioshock, and you can see that because she has a handmade doll of a Big Daddy in her hand. It shows her on the beach, possibly longing to get back into Rapture.
2. The logo for Bioshock 2 is covered in barnacles, insinuating the city is now completely submerged in water.