If Airbnb is 10% of the market and increases revenue by 89% whilst all others stay flat, that only requires an 8.9% increase in the market.

Of course, if that happens, others would be losing "share", albeit not shrinking in absolute terms. But, in fairness, the article seems very wishy washy in its definition of "market share".

AirBnB isn't the entire hotel market.

If total revenues for the whole market are 100, and AirBnB's share is 1. Then the total revenues only have to go up to 102, for AirBnB to capture all the new revenue and thus double its revenue.

The marketshare isn't based on revenue, but total supply of rooms/listings available, so its possible to lose marketshare and gain revenue.