This might be a dumb question, but is it known for sure that Elon can't, or isn't loaning his shares to shorts? It would be a heck of a lot of income for him, and if he is full of confidence that tesla is going to succeed, it would be satisfying to collect all those borrowing fees, then have the shorts get wiped out in the end when the stock exploded upwards some day. Also, if things started looking grim, he could always recall shares and cause a short squeeze to pop the stock. I don't really believe any of this, but it is fun to consider.
Disclosure filings don't go into this level of detail, but we do know that Elon has his shares in his own margin account and is borrowing against them to finance his lifestyle and other things. Ie. He is using his own shares as collateral and borrowing cash for personal use. To the extent that the shares are used as collateral, they can't be used for anything else.
Do note that this is a very real risk factor against TSLA. If Another 9/11 attack started and the stock market tanked, TSLA stock price would get hit harder than most other stocks due to its high beta. If the price got low enough, Elon's margin account would be forced to automatically sell his TSLA stock to cover his loans, thus kicking off a nice downward spiral. This isn't far fetched, it has happened to CEOs before that had similarly margined own company stock. All it takes is some external catalyst (or internal catalyst if the gigafactory blew up or something).