Good thing you declined, and you should have no trouble with Airbnb for declining, if you can prove they asked to pay off site. Can you? Did all conversation go via the Airbnb app?
(I say should, because who knows what will get us into trouble these days, both with CS, and on this forum…)
In case you haven’t already noticed, it is possible to set a max number of nights for stays. If you are nervous about longer stays (I know I am), or just don’t want them for any reason, it might be a good idea to use the max nights function. You can always state in your description that anyone interested in a longer stay can contact you to discuss possibilities. But the guest would not be able to request to book a longer stay, they would only be able to enquire - and enquiries you don’t need to accept/decline, you only need to reply to the guest by message.