The reason Airbnb is not likely to be upfront about why they're doing it, is it's very unlikely that Airbnb wants to admit they're probably being paid for it or getting some other form of special consideration for writing this.

The search benefit isn't even a long-term one: Either AMP will go away for some reason at some point, or, if the status quo is maintained, everyone will be forced to use AMP by Google's search monopoly, rendering it a null benefit to your rankings as everyone else transitions to it.