I'd say it's extremely likely to be a meteor, considering the number of independent observations, and the world wide consensus among astronomers that it was.
...maybe it was a NK missile...
It behaved exactly like you would expect a meteor of a certain size and composition hitting the atmosphere.
North Koreas Kim Jong Un can hardly even fart without CIA picking up the vibrations, and the purpose of their tests is mostly political, it makes no sense to do secret tests.
There was 1 missile send up (without payload), and 1 underground nuke blown up in NK the last months. Before that, I think it was over a year since they did anything major.
Also, why would they test fire it over china? the Chinese would very much not like that, and they are pretty much the closest thing NK has to an ally.
And above all, why would they fire a _live_300kton_nuke_ to explode over russia?
A ballistic missile travelling across china would be noticed... you need a very big conspiracy to cover that up.
A malfunctioning missile will not travel 1000s of kilometers before blowing up. specially not in opposite direction of where it would be send.
There is no sense in doing "tests" with live nuclear ballistic missiles. No-one has done that, ever.
I'd list a bunch of other reasons, but I think this is enough to say that theory is extremely unlikely.