Depends on the jurisdiction. In New Jersey, for example, "even when a job is terminable at will, a promissory estoppel claim can arise from rescission or revocation of a job offer 'where there is denial of a good faith opportunity to perform after a prospective employee has resigned from an existing position in reliance upon a firm job offer.'" But in New York, courts "have rejected such a claim as a matter of law, declining to make a distinction between the time period before and after at-will employment begins."

https://www.natlawreview.com/article/can-employer-legally-wi...

I don’t agree with the intensity of the GP’s claim but the relevant concept is promissory estoppel.