There are still two classes of problems unaffected by this change.
1) As is popular with gifs, there could very well be multiple of these autoplaying videos on the same page, probably even multiple fitting in the same viewport area. If they had sound, these would interfere with eachother.
2) My understanding is that many listen to music from some source unrelated to the webpage containing these autoplaying videos. So even if there's only one video, its autoplaying sound could easily mix with my tunes.
Doing autoplaying sound is hard, but with gaze tracking and knowledge of system sound usage status, I think a pretty good solution is possible.
You cannot easily control the volume until the video starts playing; not even when the video is still loading.
Luckily on newer version of Android there is always an option to quickly access the media volume after you press the volume button once, but it's still unintuitive.
The only explanation in the post is "the culture of the GIF now trumps the file format. With Project GIFV, Imgur is reimagining the looping GIF video with all the richness it deserves as a key piece of Internet culture." Is the idea just that people want soundless looping HD video, and they're accustomed to uploading GIFs, so Imgur is going to accommodate them by doing the conversions server-side?
I guess this is an explanation, but it's a surprising one. For instance, it doesn't really fit well with the observation that people uploaded GIFs of content that was obviously native video and not connected to internet GIF culture, e.g., sports videos.