@Fran-and-Mike0
Have the guests actually asked for a refund? I wouldn't discuss unless the guests bring up the topic.
The agreement binds guests to your stated refund policy of strict - no refund is due; unless through your goodwill or you perhaps offer to part refund on the basis that you obtain other bookings for the newly freed dates.
You can leave a message on airbnb in summary to state they wish to leave early on account of the availability of their long term rental, to leave an 'audit trail' as such.
Guests trying to extort refunds or dealing with bad reviews are big topics on their own, but first, don't load yourself with problems that might not exist.