Of course an employee handbook will hold up in court...for you sheltered folks (young and old) who have never worked outside of aviation, most employees in America just have a company handbook. If they follow it, they can reasonably expect to not be fired at random...and the courts will back that.

Also the company has to be consistent in it's application of the policy...if it twists the meaning for one employee, but not others, it will lose that lawsuit too.

Most large companies don't try to play these games, too much money to lose. But pilots, especially at the entry level, are notoriously inexperienced and afraid to make waves. Some of this is for good reason...leaving an employer under contentious circumstances may hurt your future job prospects in this industry.