Then your sources are mistaken, or imaginary.

The long range missile
failed. The rest landed in the water, as they are supposed to.

The vast majority of the time, the US missile defense system fails to
strike an incoming target, even when the operators know exactly when the
test missiles are launched, from where, and what their trajectory is
going to be. What's the likelihood they'll hit a damn thing when they do
NOT have all that info?

As far as a test warhead from Korea landing in Alaska, that's just plain
kookery.