As someone who had one goose with just chickens and ducks for years, I can tell you that what you were told is not correct. And Metzer Farms definitely doesn't keep just one goose with a flock of chickens, so they don't have any experience in this arena. As R2elk said, they're selling something to you. Of course they're going to tell you what they can to make a sale.
And what he said wasn't totally wrong. Can a goose live alone with chickens and be healthy and do 'okay'? Sure. Will it have bonded with those chickens, due to not having an appropriate flock mate to bond with? Again, sure. Will it be as happy as it would be if it was living with another goose? Nope, not even a little.
And they don't just stop alerting to danger because you have two. You don't get two geese together and now they're not going to alert any longer. Even if they were alerting just to tell their companion about the danger, not necessarily to tell the chickens, the chickens are still going to hear it, and so are you.
I have eight geese. They still alert to danger and I and the chickens and ducks still hear them, even if the alert wasn't meant for us.
Did you read the story in the first post on this thread?
Let me ask you this: Do you believe, in all honesty, that a lone goose kept with chickens that don't act the same way it does, that don't speak its 'language', that don't share its behaviors, that don't have the same diet, and that can never be its mate is just as happy as a goose that has another goose to live with? A companion that DOES share its behaviors, language, and, if it's the opposite gender, can be its mate to raise offspring with?
I really don't think the two geese in those scenarios are equally happy.