Nothing you said contradicts what I said. The scenario is that you make two trades - a buy at $10 and a sell at $20. The buy is broken but the sell is not.
Now it's as if you sold but the buy never happened (i.e., you sold stocks you didn't own). That's a short.
Now it's as if you sold but the buy never happened (i.e., you sold stocks you didn't own). That's a short.