It's telling that the author doesn't list all of the file sizes. The GIFs are gigantic but the AV1 files are LARGER than either the H.264 or VP9 versions in every example. If we wanted to replace GIFs you'd want to go with something closer to a comparable level of support and at this scale there's no reason to use a format with no hardware support and limited client support in general: https://caniuse.com/#feat=mpeg4…
um, I think at least mpeg4 and webm decoders have been emscriptemed to run in pure JS. https://jsmpeg.com/ http://libwebpjs.appspot.com/vp8/webm-javascript-decoder/ And I think this: https://developers.google.com/web/updates/2018/08/wasm-av1 So I don't think your "_any_ support at all" comment is correct.
To put this in perspective, I have a 9 year old MacBook Air at home which I use for testing. If you look at 720/1080p video on YouTube, even that ancient hardware it takes 5-10% CPU to play H.264 content. I have a 2017 desktop at work with 4.2GHz Core i7 which still takes almost 100% of a CPU to play 720p AV-1 and ~60% to play VP9, or ~1% to play H.264. That's a really big difference for something like a GIF successor which will be widely shared, often with multiple visible at the same time, and people will expect to just work even on hardware which is more than a year old while still leaving capacity to do other things.