Not sure what your metre comment is about, but it turned out that definition wasn't accurate enough to confidently state the speed of light, so the speed of light was set to 299792458 m/s and a metre is consequently the distance light travels in a 299792458th of a second.
(The second is in turn precisely defined by particle physics.)
It's a reference to Justin_K's earlier statement "a Knot is standard for navigation world wide because the unit is standardized on earth's size".
If that justifies knot then it also justifies kilometers, because it too was originally standardized on the earth's size.
Neither are exactly the size as originally defined.
A modern knot is 1852 meters. One minute of latitude is "about 1,855.325 m on the WGS 84 ellipsoid" says https://en.wikipedia.org/wiki/Nautical_mile. The difference is 0.16%.
(The second is in turn precisely defined by particle physics.)