If "top winery" means "largest winery", as it does in this story, I don't see how it says anything about the ability of tasters to tell the difference. Those who made such claims probably weren't drinking Gallo in the first place.

If people who can tell the difference are a big enough demographic to sell to, then they are employed by all wineries, regardless of quality. But an alternate explanation is that Gallo was tacitly in on the scam - they got as much PN as Sideways demanded, without moving the market.