Board Thread:Questions and Answers/@comment-27511601-20170413023248/@comment-679780-20170413115233

According to Ian Flynn, head writer on Archie Comics, the game takes place a few years after the main timeline; however, former Sega Europe community manager Kevin Eva states that fans should consider the game an alternate universe due to its troubled production, but notes that the canon is essentially in constant flux at any given time for Sega's convenience.

So you can say SC:tDB takes place in the near future of Sonic games, just unspecific how soon it takes place or how canon. And given Penders Sega/EA lawsuit that brings the canon into even more questioning but fans regard the Nocturne and the Argus event canon since Sega has yet confirmed it's renouncement from canon.