I received two of these letters last week. I emailed Airbnb to see if they were legit. The reply seems very shady:
"This is Kevin again with Airbnb and I hope this email finds you well. I just wanted to follow up with you in regards pictures of a letter you sent in.
The letter you received is from us, however, I wouldn't be able to assist you in regards to the content of the letter as I am not a legal professional. I recommend consulting with a local attorney if you have any questions. We know this isn't the answer you wanted to hear, but we do want you to know that we are always here to support you!
Thank you for your patience and understanding!"