From the article:
DG: But in a traditional short-selling situation, you typically have to borrow the shares before you can short them.
AB: Yes, and that's true here too. But if you look at the Securities Settlement Failure data, ETFs are very oddly overrepresented, so it does look like there is some short-selling that happens before the shares are borrowed. But that's a small matter. The problem is that there is no limit to the amount of short-selling you can theoretically do while still having borrowed the shares. It simply requires the same share to have been borrowed, short-sold, borrowed from the new owner and short-sold again down a daisy chain. That's how you get these arbitrarily large short interest figures.
Can't this daisy chain of short sales occur with regular stocks? A buys a stock, B borrows the shares from A and sell them short to C, D borrows the shares from C and sells them short to E, etc. Or is there some mechanism to keep this from happening?