So why do they have them on that variant?
What do you mean? There's no 'MRM-XX+Artemis IV' weapon, and I've never noted a marked increase in missile accuracy when using MRMs with Artemis enabled, and Apollo isn't in this game.
Well, I'm blaming dev oversight if it has Artemis installed, because it doesn't do anything for that variant. Or at least it should not. Because I have four MRM-equipped mechs that at one point had Artemis IV that I never uninstalled, and I never get a benefit from it.