Thanks @JJ48 for cutting through the speculation. Too bad I looked at my mail late, I could have avoided a chat with CS. That said, your experience lines up with exactly what CS told me. Thanks again. For anyone that wants the pertinent part of the transcript here it is:
Jim
What if he doesn't want to cancel?
Airbnb Support
There is no mistake from your end, guests should have informed earlier before making reservation that there will be 8 guests instead of 6.
Jim
I mean he's going to lose his payment and not have a place to stay.
Airbnb Support
In case guest do not want to cancel the reservation. Please do let us know and our team will get in touch with the guest and will help you with the same.
Jim
So either he or airbnb cs will cancel and I'll still get paid correct?
Airbnb Support
It totally depends upon your mutual understanding with the guest, if you wish to issue refund to the guest, you can issue refund as well.
Yes, you will be paid, as there is no mistake from your end.