USDD isn't like Terra. It's not actually an algorithmic stablecoin, it's just 'His Excellency' Justin Sun messing with people. There's no redemption mechanism (which is what lead to the UST collapse) and the creation is limited to a few authorized participants - notably Alameda. Sun can restore the peg pretty much at will for a few million bucks.

[edit] To be clear I maintain you should stay as far away as humanly possible, but shorting it is a suckers bet.