There's multiple parts to animation -
* choosing what animations are to be layered together at what time, is gameplay code.
* IK and rag-doll are probably physics code.
* Actually evaluating the animation layers to get a posed skeleton is rendering code (and is deterministic; you shouldn't need a dt here to advance a simulation).
* Using a skeleton to skin/render a mesh is rendering code.
You could put the first two in your fixed-step frame loop, and the bottom two in the rendering loop.