My guess would be:
* They have in their wallets some BTC and ETH. They also have at a certain (or more than one) exchange some coins - both ETH and BTC.
* When your bitcoins were sent and the required confirmations have passed, their system will:
1. Exchange that amount (maybe minus some fees) at the exchange
2. Send you (from their wallets, not from exchange's) the promised amount.
At a later point, they will "equalize" the funds by transferring to/from exchange as needed.
But this is my guess. What they actually do there, I don't know.
and for me, this scenario is the most logical, but I think he requires a large number of reserves, which is difficult for many exchangers.
I saw this as an important parameter of the BestChange monitoring service.