More or less. If SBD is at or below peg, it will be converted using the blockchain function, which permanently destroys it and reduces the supply of SBD. We won't ever be buying SBD as this would result in selling STEEM onto the market (arguably contrary to the goal of serving all STEEM holders by supporting the price of STEEM via buying/locking/burning). The conversion function creates STEEM which will then be burned or powered up (never sold or spent) as dictated by the market conditions.