Well the whole idea of natural selection does come from the fact that there is some advantage whatever it is that is developed be it a wing or what have you. There could actually be (and there are) of thoughts as to why the wings actually gave the animal an advantage even in its underdeveloped state. One could simply look at it as something is better than nothing, ie. the partial wing did give some advantage like it says in the article like pushing the bird down to provide better traction as it was probably not originally designed for flight. In which case the half winged birds would fair BETTER than wingless birds not worse.
But there are other idea worth consideration as well. Perhaps the wings again not serving a flight purpose yet again and they were used for sexual display or territorial displays. Both seem reasonable since birds like most animals dont want to die or loose their mate. So the use of wings to make the animal appear larger to ward off either competition or predators doesnt really seem that far fetched either as this can be witnessed by animals in present day.
As for the sexual selection side the females could have used the wings as for a basis as to the male survivablity even though it might not have been an logical representation they are only animals. This also could be noted today in ordinary guppies and molliesmost of the time the females seem to be attracted to the most brightly colored male, yet this serves no real function. Or perhaps it does in which the males with the trait- either bright coloration or partial wings- are able to survive even with the disadvantage it could just prove just how strong their genes truly are and make them a more than suitable mate. Just a few thoughts with more time I'd write more but I'll wait to do that if it's needed.