The paper this article is based on is from 2009. I’d argue that’s against rule 5.

I tried it on these platforms:

• Twitter: it works
• Mastodon: it works
• tumblr: it works
• Reddit: it doesn’t loop, video must be at least 2 seconds in length, not allowed in comments
• Discord: not autoplay, no loop
• Lemmy: no video hosting

In that case the only people that can answer the question are the engineers from those platforms.

Yes, but if you tried to share that mp4 on other platforms it would be treated as a video, and that’s why gifs are still relevant.

mp4 is a video format, the key differences with animated images being autoplay, looping and maybe transparency.

Of all the formats you mentioned these are supported on popular platforms:

• Twitter: gif
• Discord: gif
• Mastodon: gif
• Reddit: gif, apng
• Tumblr: gif, webp
• Lemmy: gif, apng, webp

That’s why gifs are still a thing.

The fact that GIF is still a thing in 2023 is baffling

As opposed to what widely supported animated image format?

Your link talks about the element zapper, the icon that looks like a lightning. The instructions above talk about the element picker, the icon that looks like an eye dropper.