Picture abhi baaki hai, dost! :P
On a more serious note, something seems to be off with the article:
The total Fed's balance sheet even after all the recent QE is ~$8-9 trillion
https://www.federalreserve.gov/monetarypolicy/bst_recenttrends.htm
Why should this be captured in the total Balance sheet of the Fed?
I don't think the article is saying that this is unpaid debt. We just don't know where this $21 Trillion has gone (between 1998 - 2015)