it can't be created ex nihilo

Actually, it is. When SBD is created, its initial backing is supplied by the post's reward STEEM. Any fall in the price of STEEM will result in a rise of the virtual supply (and conversely, any rise in the price of STEEM will result in a fall of the virtual supply).

Basically if you have SBD, it's like owning the backing STEEM with two differences:

• Any losses you get from a decline in STEEM market cap are socialized by creating some more backing STEEM, essentially all other STEEM holders subsidize 100% of "your" losses.
• Any profits you get from an increase in STEEM market cap are socialized by destroying some of the backing STEEM, essentially all other STEEM holders split 100% of "your" gains.

This is how leverage works.