Yes, the company can do a secondary offering after its IPO, and it's pretty common. I think the hype around the IPO will help them do a secondary next year, so in that sense the low initial price is like a marketing tool for future stock.

Also, Airbnb (like other tech companies) is issuing quite a lot of new stock in the form of RSUs used to pay employees. This constant dilution benefits from the high stock price.