The calendar synchronisation is misleading as the calendar doesn't sync, before a booking is accepted.
According to bookings help, the calendar only sync's 2-3 times a day. Thus a booking made on Airbnb may result in a double booking if a guest books a few hours later on Booking.com
You will then have a hassle of contacting the help centre to cancel the double booking for which you will be charged a hefty relocation fee.
Airbnb do it better, at least they ask for your acceptance of a booking before locking it in.
I recommend you phone them and not send messages.
You may not get the outcome you expect but you will at least get an "immediate" response