Well, you evidently realised you'd made a boo-boo (do people still say that nowadays?) but I think posters are being a bit hard on you ... we all make mistakes and then make them worse by trying to get out of them! Our first Airbnb as guests we were cancelled by a host, but that was 2 weeks before we were due to travel, not 4 months, and I certainly wouldn't have written a headmistressy letter like the one you received.
If I were you, I would write a sincere email to the guests, acknowledging your mistake and apologising profusely. Sometimes, when we feel we've been harmed or inconvenienced, we just need the "perpetrator" to acknowledge how we feel and show empathy. So ...
Please accept my sincere apologies for the inconvenience caused by my cancelling your reservation. I do completely undersand how you must feel and I really regret that my inexperience with Airbnb caused you this extra trouble. I very much hope that you have been able to to find suitable accomodation for your trip and will certain;ly take to heart your advice on more carefully considering my availablity in future.
With all good wishes
I think a gracious apology never comes amiss - and will make you, and perhaps even the non-guest feel better. After that, you've clearly learnt your lesson - put it behind you and move on!
BTW, we're NEVER passive-aggressive on this forum ... aggressive certainly, but we don't do the passive bit!