If they don't send such noticed then years later somebody could claim they don't enforce their trademark and thus can loose protection.

I'd say your use is sarcasm of the larger industry and protected use. A hypothetical airbnb-sucks.com containing an anecdote could be protected as free speech.

Looks like a pretty standard bulk email in a larger process (some clerk going through hundreds of matches in an Excel table).

Airbnb could take the issue to court. Or file a motion with ICANN(?) or your domain registrar to take over your domain. That means a third party will have to establish if there's really 'likelihood of confusion among consumers'. Clear no.