There are so many threads about how, where and when the Roms were discovered that I'll let the mods put this wherever they want.
Apparently the Vulcans know that Roms and Vulcans are related right? The Vulcans are part of the Federation, right? Now aside from any current status (i.e. military capabilities, government etc) wouldn't the Vulcans have been obligated to offer up any and all known knowledge of the Romulans? I'd be really pissed if the first time I encountered them my first officer said "Oh yea, we forgot to tell you. We're kinda, sorta related to these guys. Oops. Our bad."
So (IMO) the Federation (at least in this movie) could have had some knowledge of the Roms and not the complete oblivion that was in the original time line.
Apparently the Vulcans know that Roms and Vulcans are related right? The Vulcans are part of the Federation, right? Now aside from any current status (i.e. military capabilities, government etc) wouldn't the Vulcans have been obligated to offer up any and all known knowledge of the Romulans? I'd be really pissed if the first time I encountered them my first officer said "Oh yea, we forgot to tell you. We're kinda, sorta related to these guys. Oops. Our bad."

So (IMO) the Federation (at least in this movie) could have had some knowledge of the Roms and not the complete oblivion that was in the original time line.