Having not read the above article yet, the farmhouse style to me is not so much a style, but a brewing ideology. For me, it includes the idea of what would be available to a farmer/brewer from your region. Although not essential, that is, I would think, the purest sense of the term "farmhouse." A mixture of locally available grains, water, adjuncts, and hops, which I would venture to say should be balanced toward spicy. Another factor almost synonymous with the farmhouse style for me is the use of characterful, "wrangled" yeast. Whether or not this is a Brett strain, a saison strain (arguably just a recaptured and mutated red wine yeast), or a mixture of the above with bacterias, I believe the spicy, phenolic, rustic character is paramount to creating a good example of a farmhouse beer, wherever you are from.
In regards to the differentiation between European and American styles, I don't see a huge distinction in theory. Commercially, there certainly is. Spotted Cow from New Glarus was one of the first American beers to be labeled with the farmhouse moniker. It really is just an unfiltered cream ale sort of thing, brewed with flaked barley and corn to roughen the edges. On the other extreme, an example such as Jolly Pumpkin's Dark Dawn is a relatively big, rich, sour stout. Who is to say that this is not a farmhouse style as well?
But what do I know? I am just a piddly homebrew.