As has been said the VAST MAJORITY of people if not all, jab their jills when they see them starting to come into season, or a bit after they are in.
Your vet is expensive and doesn't know much about ferrets at all, just like a lot of vets.
The problem is, why do they not say something like 'I'm not sure i will look it up" rather than just making up some complete rubbish.
Seems like it's a vet thing.