Does a guest alteration request override my Strict cancellation policy?

I had a guest booked from July 1 - Aug 10. I have a “Strict” cancellation policy. She messaged me yesterday 6/17 that they bought a house (why they were booked to stay with me) to close sooner than later, requesting a July 23rd new checkout date. I accepted the alteration request as 7/23 was over a month away. It refunded her those nights not stayed. Today she updated it again to checkout 7/18…. She told me that Airbnb policy told her the first 30 days were non refundable. When I accepted the alteration, it refunded those days too and now we are only getting paid 17 nights from July 1-18. I (ignorantly) thought that accepting alterations would abide by the reservation policy on Airbnb and that any refunds outside of it would have to be issued manually. Should I have denied the request? How does the guest change the reservation without getting money back they aren’t entitled to due to the cancellation policy? So confused here as a host. Appreciate your help!