Former aerospace engineering here. Nautical miles are the standard unit of measurement for navigation. It's logical that ordnance developed for space use would be quicker to develop to using all the base calculates we use on Earth, then adjusting those related to atmosphere, pressure and gravity.
The range could be anything because there are many different effective ranges that active ordnance has. There is usually a minimum-arm range as in the damn thing isn't going to explode too close to you, and there are often effective maximum-arm range as in the damn thing hasn't hit it's intended target needs to be permanently disabled so when it hits/lands it won't explode. This would be absolutely critical in space because.. as you say, newtonian physics, and the damn thing could hit any unintended target in 5 weeks time. There is also an effective communications range, which would be much lower in space because cosmic radiation would be a real issue on something that size where you can't have a massive multidirectional raydome for two-way coms, then there is the range after which the propellant is effectively exhausted and it cannot re-orient and re-direct itself, which may be more or less than effective communications range.¯\_(ツ)_/¯
But I don't know the context how it was used. But sure, it's a TV show so they need to simply things. Most people aren't engineers and most people don't know how missiles work.