Agreed that they're different. One is an absolute distance, the other involves geometry of the planet we live on (which varies a bit by latitude, but not that much). Given that we're talking about naval weaponry (so source or target not far off the surface), line of sight will be well under 20 miles - or at least it is for ship-to-shore HF datacomms. That goes up as either source or receiver or both gain altitude.
The author likely expected we'd all have a grasp of that, hence the 'whereas'.
That said, rail gun projectiles are kinetic energy only - and I have to wonder how much 'punch' they would have at extreme range, given that they'd have to be on a parabola to round the limb of the earth. Standard calculations don't really apply as with other naval ordinance the energy is explosive, contained in the shell.