It's not necessary to re-render the animation in 4K - in fact, the majority of both live action and animated 4K Blu-rays are just 2K upscales. The real benefits come from HDR.

I would have thought they would render it at something high like 4K in the first place, since it's easy to downsample to 1080p? Unless, I suppose rendering is expensive and if you know you only need 1080p then maybe you wouldn't?