I was curious about this too and thought it must be a massive GIF. However it's not actually a GIF, it's an MP4 video[1], I wonder why it's mislabelled as a GIF.
Yep. Imgur's GIFV [0] from 2014 is the earliest mainstream example I know of.
In the cultural consciousness, a "GIF" represents a particular medium and form factor, which has become decoupled from its namesake file format to deliver the same experience more efficiently.
ah, yes, I see, it's the no-audio that made it a 'GIF'. Fair enough. But annoying because it automatically removes seek and pause options if it assumes it's a GIF. doh.
[1]: https://video.twimg.com/tweet_video/FV3mdpEXoAIrkA1.mp4