Realistically, Jujutsu is a pretty new project and there are many features to add and lots and lots of code that needs to be written. There is a lot of talking that needs to happen, in other words. So, most of the developers and committers actively hang out and discuss things in Discord, yes, because it's convenient, and fast, and allows all the interested parties to be alerted. I say this as someone who started hacking/working on Jujutsu recently; having Discord has been nice for such a new project.
The reality is that the project is in a part of its life where active discussion and feedback with the userbase is pretty valuable. So, you just go where the users are, and Discord is a big place for that.
GitHub Issues and GitHub Discussions are also actively used by a lot of people, and you can just post things there. Every major committer watches those venues as well, AFAIK, I know my notifications are set up for it.
Over time, assuming things are wildly successful, you'd probably have a lot of different venues to discuss things. I would view Discord mostly in the context of a new project that wants feedback, in that regard.
Too bad support/discussion happens entirely on Discord.