Yup, it's not a reboot. Interestingly enough, Sega's doing more by letting two continuities co-exist than a LOT Of companies have done regarding their "rebooted" franchises, Konami included.
That being said, hearing this is being made by ex-Naughty Dog employees, and that it looks like it might focus more on platforming/exploration than balls out super speed, this might actually be a good game. I'm super critical regarding the Sonic games after the Genesis era because they never felt right to me(not just the 3D ones, but the 2D ones, like all those Dimps made ones). Especially the hard on for super speed in the later games. Though his name's "Sonic" and he's been advertised as being fast, I always felt Sonic's speed was more about controlled speed and momentum based physics over "OMFG! Look at everything wizz by like a blur! Derpy derp!!". Hell, if an American studio can accomplish making a good Sonic game(where Japan can't), so be it. Last time Sega used American programmers to help them with Sonic, we got Sonic the Hedgehog 2!