The court allowed Hoffman to recover under promissory estoppel as a cause of action.
The elements of that cause of action are: (1) The promissor made a promise that the promissor knew, or should have known, would induce reliance by the promisee or third party, and (2) that did induce such reliance; (3) injustice can be avoided only by enforcing the promise.
Elements (1) and (2) were clearly fulfilled in Hoffman, and the court held that (3) was also fulfilled. Reading the full details of the case may well convince you that the court was correct.