Forgive me, but I fail to see how expecting video/voice conferencing software to actually be capable of carrying video/voice could be described as a fallacy. It seems to me like that is kind of a core functionality to any software trying to fulfill that role.
IRC has nothing to do with the subject, and while XMPP/Matrix are promising they are still a long way from being able to talk someone without significant tech expertise and who has never seen them before into jumping onto a call in five minutes or so without touching a single setting. That is the fundamental part of Discord, Teams, Skype, or Zoom that matters.
Lemmy isn’t exactly voice conferencing software, so I don’t know why you would want to collaborate on software development work with it as a forum. As for documentation, a static site is probably the best place for that, although in this case keeping it off the clearnet was presumably a core consideration.