I wouldn't say they're promoting it as "a co-op game", they're just promoting the fact that it has extensive co-op/multiplayer. Most of the marketing has been about the fact that this is a brand new classic Sonic game. I'd also wager that the main campaign probably presents logistical issues or hiccups that the much-simpler battle modes don't. The battle modes are 1 minute long rounds in relatively short levels, as opposed to classic Sonic levels which can take up to 8 minutes. There also are some obstacles "designed for multiplayer", in the sense that they function with multiple players at once. The big loop at the end of Bridge Island Zone Act 2 and the hanging ring dispenser thing are good examples.