That is probably referring to options, not short sales. During a short squeeze the effect is basically the same, whoever sold "naked" call options will have to

That is probably referring to options, not short sales. During a short squeeze the effect is basically the same, whoever sold "naked" call options will have to buy shares (or buy back the option, or buy some other option to offset the loss) to cover their position. Like short sellers, people who sell naked calls will start facing margin calls from their brokerage and will be forced to close the position at whatever price they can find (either for the options or the shares they would have to deliver if the option is exercised).