@John-And-Lara0 Unfortunately, Airbnb reps are ill-trained know-nothings.
What will work for this situation is to have 2 listings- one only for the main house, and one for the main house plus the outbuilding. Syncing the calendars will then work fine, because if someone books with the annex option, it will block the main house only listing, and if they book the main house only listing, the one with the annex option won't be available, which is what you want.
Then I would put wording in the main house only listing regarding the other option, in case guests looking at the main house listing think, "Hey, that would be a good idea, for grandma and gramps to have a place to get away from the hustle and bustle since they go to bed earlier and get up earlier than everyone else."
Of course you would only show photos of the main house on that listing, and photos of both on the other.