Correct, the idea is to have SBD pegged to US$. How to do this? Indeed interest rates. In addition, witnesses can also set a discount to Steem value used for calculating the rewards. It becomes even more complex, because this Steem value is not the internal market or external market Steem value, or at least not the same parameters. https://steemit.com/sbd/@timcliff/attention-sbd-converters-please-read-important-information-about-sbd-steem-conversions