He has left the topic open to contributors’ interpretation but provided some guidance, including whether we think there is a skein or theme to usage of the term.
I’ll do an uncharacteristically short post and say, in my experience, almost all farmhouse beer is saison, a riff on Saison Dupont or Silly or one of the other old-school Wallon beers.
Virtually every farmhouse beer I’ve had has the typical yeast signature of those beers. Some are tart, but so were numerous of those beers.
So for me, farmhouse means that style. I would suggest the ubiquity of the term today is due mainly to an influential book, Farmhouse Ales: Culture and Craftsmanship in the Belgian Tradition, by Phil Markowski (with two contributors), published about 10 years ago.
(The image of the cover is from the amazon.com listing, here). Markowski, a brewer formerly with Southampton Publick House on Long Island, NY, was an early promoter of the related styles of saison and bière de garde, and treats of them in the book.
Do I fancy the beers? Not really as I find the saison (Belgian-origin) yeast signature unappealing and beers using it tend to taste rather similar.