I never implied this either. "For example, if their dog is vicious or snappy or difficult to control then i'd hope they wouldn't put the dog anywhere near the guinea pig in the first place...." In my previous post. In other words, if you can't teach the dog to behave otherwise - don't attempt it.
While I think we are both after the same thing -- ensuring the pigs are safe and the dogs are happy too -- I still maintain that we cannot ever be 100% sure that we have taught out the instinct, or as you say, taught "the dog to behave otherwise." All it takes is one time for the dog to prove that we were mistaken, for it to be too late for a pig. The dog can be peaceable for 999 times, but who knows what could trigger it? Maybe it never happens. Why take the chance?
Keep them in an entirely enclosed cage. Decide whether the dog should have access to the room they are in.