Selling stock you own and selling stock you borrowed from somebody else with the obligation to return it at some point are pretty different

Yes, the latter is far riskier and obligates you to buy shares later, meaning you have to have more conviction in your investing thesis.

(Disclosure: I’m short $NKLA after Hindenburg published. I believe it’s more likely going to the teens [and possibly worse] in 2020 than sustainably into the mid-40s. If it holds $30+ through 2020, I’ll probably cover.)

For the seller it's different, to market it's just a sell.