This scene is a good example of my issue with metalbending in Korra. In the original series, Toph had to be in physical contact with the metal, because it was much harder to bend than actual earth, because she was bending the impurities within it.
Meanwhile, the metal bending in Korra is basically the opposite, they actually bend with far more precision than anyone bends earth with, which doesn't make any sense.
It's kinda in the same vein as the lavabending, where instead of applying any actual logic to it, they treated the bending as pure sorcery. Korra doesn't learn it from anyone, she just has it, because she has more Bending Midiclorians than everyone else, I guess.