How can Steemit back the Steem Dollars with US Dollars as promised?

Steemit Dollars are said to be "about" on par with the USD.

Surely Steemit must have lots of USD nominated cash income in order to be able to back up their claim that the Steem Dollars are worth on a 1:1 ratio to USD. Basically they will then be acting as a central bank which has decided to peg their currency to the USD. In this case, however, they have already set in stone the amount (and increase) of Steems, so I don't really see how they could manipulate the exchange rate in any other way except to just simply have lots of dollars and buy up all the steems at a pre-set price.