Weird. The backend actually went out of its way to extract a static PNG thumbnail and uses that as the expando source image rather than using the post's webp image hotlink.
- Thumbnail URL: https://lemmy.sdf.org/pictrs/image/d799b1f5-6f22-481e-9118-834ffe4dc8de.png?format=webp
- Post URL: https://mathiasbynens.be/demo/animated-webp-supported.webp
Conclusion: hotlinking animated webps does not yield an animated image as far as I can tell