You are correct, antibiotic and antifungal products will not prevent Equine Influenza because it is a virus. I don't know anything about Australian law as it concerns drugs vs. supplements, so I can't tell who why they can make this claim. But it does bring their whole spiel into question doesn't it?
As for lung capacity, it can be measured---it's just not something you can do in the average veterinary setting. It requires a special setup. I don't know how much of this claim is based upon fact/research rather than personal experience and we all know that personal experience can be rather biased and not terribly accurate.
Licensed Veterinary Technician