From the post it's unclear whether Tesla made a signed offer, or just sent the offer without signature. If the latter, there's probably not much that can be done about it. That kind of thing actually happened A LOT last year, and LinkedIn was full of people warning you to make sure you get a signed offer before you give notice in your current job. In the middle of the pandemic, many people went unemployed because of this.
Now, if the offer was signed, I'd imagine the contract had some terms and both the employer and the employee have to abide by the terms of the contract. As a candidate, you can reject an offer in the last minute, even if you accepted it, as long as you didn't sign it. But once you sign the piece of paper, you've got liabilities. There'll be a notice period and other things. And the same goes for the employer. I guess even in the US companies have liabilities towards their employees?
I'd think if this candidate passed an interview with Tesla, he won't have much trouble finding another role. It's always more difficult when you're unemployed, but you can always build leverage by applying to multiple companies, get multiple offers, and let them compete for you. Then again, NEVER turn an offer down until you're all signed with one of the potential employers and ready to start. If you can, don't even turn the other offers down until you've already started the new job. And when you decline the offer, do so in good terms making it clear that you'd like to consider working for them in the future (you can be fired anytime).