Comment: [Russia has a maritime border with Japan less than 50 miles wide at some points.]
You are referring to the pennisula that Russia took from Japan as a result of WWII?
[The fact that Japan was the aggressor in the Sino-Japanese war of 1895 is irrelevant.]
It is if you completely disregard the often repeated and intentionally highlighted aspect of "threat."  It seems you are cherry-picking "neighbor" and ignoring "threat."  AND the point about not being neighbors like NK and SK are, ie. in range of even WWII artillery.