The original statement is generally correct. Employee handbooks don't have contractual status (i.e. they can be amended at the company's discretion) but do have legal status insofar as employee's will be expected and, in most cases be contractually bound, to abide by the terms of the handbook.

It can bind employers, too. In most states, the answer to "what happens with unclaimed vacation pay if I quit?" is "go see what it says in the employee handbook."

Most (if not all) explicitly say they are not to be mistaken (in any way) as legally binding contract. With at will employment, this seeming nuance is worth keeping in mind. It means the employee cannot rely on such a handbook, in court: the employer has explicitly prevented this use. As you correcty point out, an explicit contract may require one or both sides to live up to the handbook. But such employment contracts are not considered the general case, in the US at least.