He's kind of got a point -- I generally agree with the author that historically, most saisons would have been well attenuated, and phenolic from the Brett in the barrels. That's about the extent of what a saison was, historically. They may or may not have been sour... but many/most probably were. But traditionally, the pitching yeast didn't matter. Only the weird critters and barrel character did.
Both historically and today, "saison" is about as much a "style" as "lager" is a "style". The boundaries are super broad. Keep in mind, the Belgians don't really care about "styles", as much as they care about quality and "drinkability". Give me some wort, and I can throw some weird enough critters at it to call it a "saison" if I wanted to. Or cheat and add amylase or Beano (but I wouldn't actually do that unless strongly coerced).
Genetically, all of the commercial saison yeasts are closely related derivatives of WINE yeasts. Which makes a lot of sense, given that a lot of saisons over the years were no doubt aged in old wine barrels. They have their own branch on the genetic tree, side by side with wine yeasts, but far away from any other beer yeasts.
So..... is it a myth? Yes and no. Both. I do feel like I could make a "saison" from ANY yeast.... as long as I could ALSO add Brett or something wild to it as well.