Real missiles can miss, malfunction (the missile), be intercepted, not have a nuclear warhead in the first place, experience a warhead malfunction, and—even if they hit somewhere in the area of Hawaii and detonate a nuclear warhead—leave much of the state intact (Hawaii is, after all, about half as far across, east to west, as the continental US.)

The very last may not produce an all-clear in the short term, but it's likely to at least require lifting the “missile is likely to hit in our area soon” alert in favor of a very different emergency posture, which is a similar need.

Firstly, why would you give an alert if everyone is going to die and there's nothing anyone can do? The reason for the alert is that most people (~98%) will be outside the fatal blast of a N. Korean Nuke assuming that it even hits it's target, which I'm not sure the likelihood of anyway. You need the all clear for the surviving 98% (or 100% of people on a miss or false alarm) who will be continuing to get things together after the attack.

If the US is going to exchange nukes with China or Russia, they could decide to wipe out Hawai'i. But N. Korea has like, what 5-6 nukes? They're not known to be accurate either.

Except for Honolulu Hawai'i is mostly composed of small population centers separated by miles of ocean or valley walls. If N. Korea decided that Hawai'i really had to go, I'd guess they could use their arsenal to kill/injure max: 6 missiles * 20k casualties = 100k casualties total, which would still leave 1.1million people to hear an all clear.