Yep that worked. What I did was go to airbnb.com (make sure you're logged in) and click on the logo top left and select messages . On the messages page there should be a dropdown menu somewhere near the top where you can see how many archived and unread messages you have. This will give you an idea of which ones you need to click on to view. (Obviously I have read these on the app but for some reason the app and website don't communicate properly).
Once you click on the messages that were unread or archived etc then you can go back to the app and refresh the messages page and the dot should go away from the message icon.