That's a very unfortunate situation: being an Airbnb host, from some point of time I started receiving messages with other guests for listings that I have no relation to.
From one side, I can simply ignore it, but from another - who can guarantee that communications with my guests for my listings don't go to other users?...
Having created a support case in Airbnb, I simply got a message that it'll be addressed within 24 hours, irrespective that I indicated that it's a breach of confidentiality.
So, being a host or a guest, please be careful with information you circulate in the Airbnb messages. "Better to be safe than sorry" as an old proverb says