I suspect that you're analyzing the frame trace wrong and thinking that since pre-skinned data is showing up in the draw it's being sent from the CPU, but feel free to send me a renderdoc capture or something and I'll take a look. Compute skinning and vertex skinning are both competitive; for games with many passes, zpre, multiple shadow passes compute skinning can beat vertex. Compute skinning is used by Frostbite and most other large engines too.