I suppose the point is that's there's little difference, practically speaking, between this situation and being fired 5 minutes into your first day on the job.
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.