well, it doesn't need to be backer only, yeah. but still, that way could work, considering they're apparently planning a shovel knight style expansion release. people who got digital only for $28 get just that, and those who get physical for more get digital from get go+all the free addons as they're released, and once it's all finished and complete physical version is printed they get that too. that way they don't have to wait additional year or two to play the game, creating unnecessary hype that won't be able to live up to reality, or disappointment and tiredness from constantly delayed content.
there is literally no difference between physical and digital as far as gameplay goes, and pretty much all hardware this game's for supports such releases.