Moderation is handled by each instance's version of that community separately.
Reddit/Lemmy/etc communities differ from something like Tumblr/Cohost by also having per-community rules, and nobody has the time to moderate hundreds of communities according to their per-community rules.
It's relatively easy to keep an instance free of spam/overly blatant hate/etc, since that is a fairly common set of rules. But it's much harder to keep a "world news" style community being overran with US-centric posts, or a discussion community on a specific subject from being filled to the brim with memes, or posts that are only very vaguely adjacent. Without centralized per-community moderators, it would fall on general instance moderation to make decisions about whether a post about an Undertale hack fits in the Undertale community. That's probably going to go wrong more often than not.
You can have a website that is only moderated according to global rules with tags being a free-for-all, but you fundamentally end up building something along the lines of Tumblr or Cohost, which attracts a different audience, including those that know how to rules lawyer their way in such an environment; tagging 20 mediocre photos a day with #photography
instead of just a good one, for example. With the end of Cohost approaching, I wouldn't be surprised if some tried to build that kinda thing, but it'd likely end up having a very different vibe.
Digging into the GitLab & related discussions, the main takeaway I got is that FFmpeg's API supposedly meshes better with what Wine needs to provide to Windows code, simplifying things overall. GST is pretty heavy on asynchronous/background processing, which is normally something I'd consider good for media, but if the API you're expected to implement is synchronous then I guess it only adds complexity.