To me, there does seem to be a delay @Rebecca1603
When I’m on the app messaging with a guest, the emails seem to come through delayed. I’m not sure by how many minutes, but I assume as long as the edit function is available. Unsend seems to always be available. I assume if you unsent after editing wasn't available, the guest may have received an email.
It's also possible that they may have app notifications turned on and could've read the notification without clicking through to the Airbnb app.
You should also check the message thread to see whether previous messages from you to that guest, have read receipts. One can turn receipts off and then nothing shows "read".