Angry Birds was definitely indie! I agree with your definition that indie denotes a...fringe title, but I also disagree. It all depends how one is using the term, and it can be used several ways. Indie officially means independently developed without a publisher controlling the studio. This meant the studio had complete artistic freedom and is why indie titles could be so diverse. But now, due to language changes, indie is starting to refer to the style of studio rather than the official root definition. So Insomniac's Sunset Overdrive on XB1 isn't an indie title because Insomniac are a big developer and not a tiny bedroom studio, unlike Young Horses with Octodad. The problem with that sort of definition is where you draw the line. The official definition is usefully exact, but doesn't fit with people's perception of an indie studio. We really do need better definitions!Sorry I disagree. Worms is as indie as Angry Birds.
But for MS's purposes, Team17 are every bit as independent as Housemarque, and Worms, no matter how unoriginal, is an independent game every bit as much as Resogun (in fact moreso, because it's not platform exclusive or developed under contract to a publisher).
If you want to try this another way, what is your definition of indie? See how that matches up with other indie libraries like PSN and Steam.