[filvdg]

the author explains buying put options , not short selling. in shortselling you borrow shares and sell them , with the promise to give the shares back to the original owner in a later date (so shortsellers hope to buy back the shares later at a lower value)

[mxschumacher]

shorting means betting on falling prices, how such a bet is implemented is an implementation detail. Borrow+sell, put options or much more complex derivatives are possible.