I wouldn't bet on either side a priori. I know what I want to happen, but the legal process here is perverse.

Failing to defend, failing to adequately resource a defence of even the blindingly obvious can lead to some odd outcomes.

Personally I hope for a pro bono, well structured "piss off, this is stupid" line of reasoning from history. Ask yourself why "AirBnB" chose the 3 letter form in the first place. Thats .. prior art, a generic, or a combination of the two.

(not a lawyer. Hence my doubts this is as straightforward as I personally would like)

They chose BnB to suggest "bed and breakfast", a common term for a small hotel that was typically just an old house where the owners lived and would host, as we all know. So we know what the right result will be. But since it's a giant corporation versus a person with no resources, we may get the wrong result.