Especially when Matrix is right there, or Mattermost, or IRC, or email.
IRC: maybe, I've seen some webchat frontends that make it really easy to start. but people expect voice chat, embeds, etc so IRC doesn't really cut it anymore (which I find very sad)
The world they want is an ideal that hasn't been achieved yet.
It only makes sense to hobble yourself by not taking advantage of the existing infrastructure if your ideology holds existing infrastructure to be inherently immoral not merely substandard.
Private centralized spaces are less than perfect, but they're okay to use while you work to make something better.
The problem of decentralized/user-controlled communication is adjacent to the problem that this is trying to solve, so it's not even like "We're working on this, it's just not ready yet", they chose to use a centralized option over a decentralized one that is ready.
In this specific instance, I would argue that it is immoral to use Discord[0], as it disregards the efforts of a production ready project serving the same goal, in a different space.
[0] Perhaps not quite immoral, but pushing that direction. It certainly casts doubt on their actual aspirations.
Edit: I would like to thank you for your comment however, as it did make me stop and think for a while, and better consider my position on this topic.