AirBnB is using an extract, load and transform architecture. No mention of the hardware, data through put, whether they have a message broker/queue to ease the burden of peak volume but work.
I have a strong feeling that they could have 1.) Kept the system exactly how it is and done some performance tuning. But that's not sexxy anymore. Things are just supposed to scale. Which brings me to
2.) Moved transformation logic to its own server or multiple servers using a message broker and queue to aid the transfer of data between systems. It would have been more readable and could have been done in a mo the or less.
In summary I believe they should have put some effort in to keep SQL. Especially for the purpose of accounting because spark does not lend itself to readable logic.