It's all easy with hindsight. It was much less obvious at the time: https://medium.com/@bchesky/7-rejections-7d894cbaa084

AirBnb was almost dead when they got into YC. Nobody would fund them, and Nate had moved to Boston to get a real job.

Even after YC and with the full support of PG himself, AirBnb was turned down by one of the top VCs in the country:

http://www.paulgraham.com/airbnb.html

And that's just one public example. (Chris Sacca also turned them down.)

Empirically it would be shocking if they were the only 2 investors who had the opportunity to invest but didn't.

Sure, but those were very far from being mainstream.

Airbnb is actually still nuts. They probably aren't even to the half-way point in terms of where they need to be to be actually mainstream.