Given that ffmpeg was apparently using mailing lists before (which I didn't know at the time of writing), this is obviously an improvement. But it's still a lot of friction, and it seems that it's friction for no particular reason.
Hosting your code on something as obscure as Forgejo actively makes it hard for people to contribute. Most developers use GitHub, and only have experience with GitHub.
If I have an issue with something, I'll make a PR and move on with my day. But if making that PR involves creating an account on some obscure website and learning a new user interface just to contribute code to a single project, I'll simply pass. It's not worth the hassle.
GitHub is the most popular service by far for a good reason. People build their whole professional portfolio on GitHub, and it's well-known and globally recognized.
If you're afraid GitHub is gonna go down one day or kick you out, just mirror your repo to your own git instance. It's been done for decades, and it works very well, without introducing friction.