No, you're thinking about this all wrong, the valuation is due to all the physical assets they own.
More seriously, these kinds of valuations send a powerful signal that physical property ownership should be viewed less and less as an asset to a company and more as a burden.
Since AirBnB shifts the property ownership over to their room network, shifts up and down in the short-term rental market hurts them and provides a buffer to AirBnB. Traditional hoteliers have to keep maintaining empty rooms in the event the industry picks back up, or build more rooms if the industry is at a high. On the flip-side, AirBnB can be sensitive to local regulations and property owners might simply decide to not have their place up for rent, making availability a slight risk.
To put this valuation into perspective, Marriott International has a Mkt Cap of $21.5B They're the 3rd largest hotel group on the planet and own something like 125,000 rooms.
Airbnb claims 1,000,000 listings, but owns none. Makes money only off of the service fees (6-12%) and credit card processing (3%).
Looked at another way, this valuation says to the hotel industry "having professional employees and facilities is eating into your bottom-line", because AirBnB simply doesn't have to assume those costs.
Prediction: regular hotel rooms will start showing up AirBnB within 5 years.