Thank you.
The price feed allows for a bias so that the blockchain can undervalue STEEM in order to print more SBD to (in theory) when the price of SBD is higher than $1. It seemed to have some unintended consequences which caused more confusion (the primary concern being the smart contract from SBD to STEEM, but that issue was resolved by removing that option from the Steemit interface). Some witnesses still support using it, but in all the discussions about it a long time ago, I remember being unconvinced it was a helpful approach.
The APR is only for SBD. It’s a tool for helping increase the desirability if SBD when it falls below $1, but since that’s not the problem we have, the interest has been at $0 for a while now.
Thank you for your answers. :)