Comment by aniviacat

The difference matters, as it means that AVIF's "progressive decoding" serves no technical benefit and is only a minor ergonomic benefit: On a website, you could achieve the same effect simply by layering two <img> elements on top of each other, one with the thumbnail and one with the final image.

If layering to <img> elements is fine for your use case, then it simply means you have no need for progressive decoding.