In any event, this was the sentence in the abstract that grabbed my attention:
This morphotype is congruent with the stage I morphology predicted by developmental models, and all major predicted morphotypes have now been documented in the fossil record.
The "developmental model" they referenced was outlined in a paper by Richard Prum, "Development and evolutionary origin of feathers" (J Exp Zool 285:291-306, 1999). He based his model of feather evolution on the development of the feather, or in his words,
The evolutionary polarities of the events in feather development are inferred from the hierarchical organization of events in feather development, or, in some instances, by the physical necessity of a given structure to subsequent developmental events.
Xu et al. argue that their discoveries confirm Prum's model and that all of Prum's feather evolution stages have now been documented in feathered dinosaurs. I'm not going to argue with their interpretation, but I'd like to suggest an alternative interpretation that would be consistent with a creationist perspective on these dinosaurs.
Think about this: Prum's model of feather evolution is based on development, so his stages of feather evolution correspond roughly to stages in feather development. When these stages are found in the fossil record, they are assumed to be representative of feather evolution, but what if they are merely representative of feather development? We have examples of modern organisms that retain juvenile traits into adulthood (most famously axolotl), caused by various forms of heterochrony (e.g., paedomorphosis or neoteny). What if these various stages of feather "evolution" are actually examples of heterochrony? What if what we're seeing in these dinosaurs are variations of feather development, perhaps within a single baramin?
That's an intriguing idea. Someone should look into it.