I tried these examples on ezgif, and indeed apng manages to be smaller than webp every single time. Weird, I thought webp was almost always smaller. Is this because GIF images are already special, or apng uses better compression than png?

edit: using the same ezgif webp and apng on a H.264 source, apng is suddenly 10x the size than webp. It seems apng is only better if the source is gif

I have no idea! I actually hoped someone would show a much more comprehensive and serious benchmark in response, but that has failed to materialize.

I would guess that apng only wins when indexed colors can be used. That guess would match what you saw using an h264 file for the source.

Almost like video codecs and animated images are different niches that optimize for different content.

You're correct and I was considering adding a footnote that if you use indexed colors like a GIF then PNG can beat GIF due to better compression algorithms. But when most people think of APNG they think of lossless compression rather than lossy compression.

Indexed can be lossless when the source already uses few colors, e.g. because you want to improve the compression of an existing GIF or limit colors for stylistic choice (common in pixel art).