FIrst of all, giving a rabbit steroids is never ever a good idea as the first defense against anything – they are very sensitive to steroids and it is the last thing a rabbit experienced vet would ever use on a rabbit.
I really cannot imagine a rabbit being allergic to carpet – they can certainly have sensitivities as with any other animal or human. I can imagine they might have seasonal allergies like we do too.
But really how can you be sure it’s allergies? Probably the vet says that because they have no other ideas. As for snuffles, as Rabbitpam mentions that is just a catch all term for an upper respiratory infection and usually refers to pasturella but how can a vet just tell my looking that your rabbit even has pasturella and if they do, it’s very treatable and you would need to do a culture and sensitivity test to determine that and in my experience it’s rarely pasturella anyway.
I would want another opinion as well.