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.