How does someone make 21 moves with 19.9 seconds left to play?

The most likely answer is premoving. Your opponent can think of a move during your turn and place the piece before you have even moved - then when it is their turn, the move is made and they do not lose any time at all. I have personally seen various games finished entirely in under a second! It takes a fair amount of skill to premove that often; if you notice your opponent doing that, throw in a few unusual or tricky moves to either win material because of a bad premove or to disable their premove (e.g. an unexpected check - premoves are rejected by the system if they are illegal).