I haven't stayed at Airbnbs in the US but it has been a common experience in Europe to be instructed to say we're guests of the owner if asked. I assume this is about either HOA rules on subletting, or tax/insurance/regulatory issues.
I think it makes more sense. If you just say "I am at the airbnb", it's confusing. Maybe they don't know what unit you're talking about. If you say "guest of blah blah" there is some clarity. Eg. even if they don't know their neighbors, it's likely that the host's name is at least on the buzzer or mailbox. And if you remember your host's name, you may know which buzzer to hit at check-in, for example.
I doubt that would hold up for insurance purposes. Also, I think it's relatively unlikely that tax inspectors come around and talk to suspected guests, at least compared to the common situation of HOA/lease policies restricting subletting.
