Yeah, looked at the IGN review quickly, and they're too different games. Developed by Dimps like the Rush games and Sonic Colors, and apparently lifting classic levels entirely, almost making it a greatest hits of Sonic rather than a tribute. Plus I'm seeing Sonic using modern moves in a classic, side scrolling stage, which you DEFINITELY can't do in Generations.
Look at it this way though: Now you have two games rather than just one! Though I'd appreciate Sega not doing this misleading labeling bullshit and just make a different game entirely. Or give it a subtitle to clarify it's not the same, whichever.