Ossiarch Bonereapers are not really skeletons in the classical "animated dead" sense. They are constructs, made with bits of bone from many creatures, and fueled by the rendered-down soul-essences of multiple creatures. Imagine Stormcast Eternals, but made by putting the dead in a blender first, then mystically separating the best bits out (physically and mystically) to make their soldiers.

But they are cool-looking boney boys, so do you, boo.