He's talking about Dragon Tail, the QCB+D, not Dragon Kick. When he's sober, slide into QCB+D is interruptable, but the window to reverse seems very small, as I've had people struggle with getting around it. When he's drunk, I think it may actually be a true blockstring, which is disgusting once you've cornered someone, as you can just do slide into QCB+D, land and close C into slide and repeat, provided they don't have any bar to get out. Makes for easy guard breaks.