My guess: because the OP forgot that to decode AV2/AV3 it matters not that it is in an AVIF container, it matters that the display system (browser, etc.) actually have the codec available to it that can decode the new format (AV2/AV3/etc.).
replyWith no codec for a new compression algorithm, it does not matter if the browser can read the container, you still see the "broken image" icon on your display.
You didn't get the point of the post. I explained why and how container matters here. Not a specific container, but just the idea of container itself. Hint, it's not the browser.
replyI don't get it. And to me it wasn't clear in the post. As others have explained the container and codec are two separate issues.
replyConcrete example. Let's say GitHub will add AVIF support at some point (it's not yet handled well by the site as you can notice[1].
replyYou as user can then upload AVIF images to wherever images on the site are processed. And after that, let's say when newer codec inside AVIF will get a good browser coverage, you'll be able to start using it without waiting another decade for GitHub to pipe it through their site code.
For formats without a container, it's always a two step process. Browser coverage first, then another huge period of sites support catching up.