it’s not an exoskeleton, it’s litearlly just a steady cam harness, even in universe it’s literally just a steady cam harness that’s fit to work with the weapon.
Fun fact, do you know how you fire in the prone position with a smartgun? you lay out on your back with the gun down your body shotting from between your legs, I shit you not, it’s in the “Colonial Marines Technical Manual.”
it also covers how to properly fire the weapon from low cover positions too.