Not true. Almost all ski helmets are single-impact. If you've taken a fall and concussed yourself, you need to get a new helmet.
Basically, the foam is what keeps you safe, not the hard shell. Once you take a large fall on your helmet, the foam compresses (doing it's job) but doesn't rebound. If the foam can no longer compress on another impact, it cannot do its job. Your helmet is toast, get a new one.