I believe in Karma so could come back nicely to you!
Well, im sure they will remember you now & your kind gesture---
Slip a bottle of wine in your bag and and pull it out if you see other people have brought something.
So in short, the girls were wrong (as usual) about having to bring a gift but it was still appreciated. Glad you experienced one aspect of the Swiss: being reserved in public and rather familiar and warm in private.
BUT always better to be safe than sorry