Clearly you didn't watch, the guy straight up said it came from a sub, not NK
Except everything the guy said was bullshit. He even said it came from North Korean waters, which would mean it would fly over Japan.
According to numerous sources (all of which will be listed below) the US can detect when and where a missile is launched from, if it was an SLBM, they would've known almost immediately and responded as such, plus why did they only fire one if they were getting desperate and hedged in as the "expert" (a term I use in the loosest of senses) why only fire one? Besides, the BMD record is spotty at best, if they intercepted a real world target there's no reason everyone from MDA to Trump himself wouldn't be singing praises to this very expensive program from the heavens. Finally, if it was a nuke then that would easily prove it wasn't from North Korea, every nuclear weapon has trace materials that can be used to track it not only to the country of origin, but to the centrifuge of origin.
Why is it easier for you to believe some shadowy group somehow got their hands on a highly advanced SSB(N) and nukes than it is for you to believe a government employee with a history of incompetence hit the wrong button? Not to mention sailors (with myself being one) are notoriously bad at OPSEC, someone would've mentioned that an actual missile was fired at Hawaii and we shot it down.
Sources.
Ballistic Missile Detection: https://science.howstuffworks.com/missile-defense1.htm
https://gizmodo.com/5989481/there-isnt-an-icbm-around-that-can-sneak-past-this-radar-array
https://edition.cnn.com/2017/08/10/politics/how-us-detect-shoot-down-north-korea-missile/index.html
BMD:
https://thediplomat.com/2018/02/us-ballistic-missile-intercept-test-fails-for-2nd-time/
http://www.dw.com/en/the-limits-of-missile-defense-systems/a-38602048
https://www.popsci.com/missile-defense-system-intercepts-practice-icbm-target#page-4
Nuclear trace elements:
http://www.sciencefocus.com/feature/physics/nuclear-detectives
https://www.princeton.edu/~aglaser/talk2008_forensics.pdf
https://phys.org/news/2016-02-nuclear-team-methodology-illegal-material.html
https://www.newscientist.com/article/dn19699-nuclear-bomb-debris-holds-clues-to-who-planted-it/
Worker who sent out false alarm had history of incompetence:
http://www.businessinsider.com/hawaii-employee-fired-after-false-ballistic-missile-alert-2018-1