Yep, so there's a 100% probability that everyone's not-favorite hand-holder was able to fight all along, but chose not to for some odd reason. Sound familiar to anyone? Really, all that's missing is the Rapunzel hair. I mean, gee, at least Midna did something every now and then.
With that said, let the fanwanking begin. By tomorrow the internet will be flooded with theories on why Fi did nothing but run her mouth in Skyward Sword - and the only one that'd really make sense is Hyrule Warriors being no different than Super Smash Bros. in terms of canonical presence.