The central issue is that Tesla gave him a job offer and based on that offer he resigned from his prior job. Tesla then reneged on their offer weeks later. The damages are the lost wages due to them not following through with the job offer after they caused him to resign.
It’s relevant that tortious interference isn’t specific to employment, and thus isn’t really impacted by at-will employment. A company can legally rescind a job offer, or fire you 5 minutes onto the job. The civil claim isn’t that they can’t fire you, it’s that their employment contract was done in bad faith, and that they owe you for losses you incurred as a result of being a good faith actor who entered a contract with a company acting in bad faith.